Is it a crime to have child outside of wedlock?
Giving birth itself, regardless of the circumstances, is never a crime in the United States.
Some sexual acts which can sometimes result in the conception of a child are crimes (most obviously rape and incest). Circumstances other than rape (including statutory rape and abuse of a position of trust rape), and incest, where a sexual act that can result in the conception of a child are a crime based upon the marital status of one or both of the parties to the sexual act are discussed below.
If a legally married man has a child with another woman, what are the
consequences for that child?
1- Who will be the legal father of this child in the child's birth
It depends upon how the process is handled. The mother's husband is the presumed parent whether or not he is listed on the birth certificate. In some states, "the other man" does not have the right to bring a paternity proceeding, although the husband or the mother would. The U.S. Supreme Court has held that this kind of limitation on paternity lawsuits is constitutional.
But, if the mother indicates the biological father's name on the birth certificate (some states require the biological father's voluntary acknowledgement as well), that can supersede this presumption.
The biological father always has some process (sometimes difficult for even a non-blameworthy father to comply with) to assert paternity in cases where the mother of the child is unmarried (or was unmarried at the time of conception at least), however, except that in a minority of U.S. states, rapists do not have the status of legal parents of their biological children with a rape victim.
There is usually a statute of limitations for the paternity of a father shown on a birth certificate to be contested for the purpose of using that as a defense to a child support claim.
Sometimes another statute of limitations applies, however, in other circumstances, such as a lawsuit to collect child support brought by the mother of the child, or the child, where paternity hasn't previously been established, or for inheritance purposes.
The exact process by which paternity is established when the biological parents are not married differs from state to state and is the subject of several separate state specific answers at Law.SE. The process is frequently different when the mother is married to someone else than it is when the mother is not married.
Some states (including California) permit a child to have more than two parents in some circumstances, and to have two parents who have the same sex in some circumstances.
The fact of a genetic parent-child relationship is, as a general rule, neither necessary, nor sufficient, to establish a parent-child relationship, although it is a very important factor to be considered by a court.
There is a special body of law governing paternity in cases of "assisted reproduction" (such as artificial insemination or when donated eggs are used or when there is a surrogate parent who gives birth to a child after having a fertilized egg from a different biological mother and father implanted in her). The majority rule in these cases, where assisted reproduction is intended and all parties to the process consent, is that an assisted reproduction agreement of the parties governs paternity. But not all possible scenarios have been the subject of clear law, and there is not uniformity among states regarding the relevant law.
For example, there is not much guidance regarding the legal considerations that apply in a case where a man artificially inseminates a woman causing her to become pregnant without her consent or knowledge at the time that this happens, and only subsequently learns what happens, or when the sperm of an intended sperm donor is substituted for sperm by another donor, in an artificial insemination procedure to which the woman otherwise consented.
Termination of parental rights and the parent-child relationship is something that, once established, is hard to effect legally in most cases and is a situation in which an indigent defendant facing a parental rights termination has a constitutional right to counsel. But termination of parental rights in adoptions, when the paternity of the father hasn't been established legally, can be done much more easily.
2- Can this child use her/his biological father's last name?
A child can have any name agreed to by the parents designated on the birth certificate. It is customary to assign a child the father's last name (unless no father is listed on the birth certificate or publicly disclosed), but in the United States, the issue of what someone is named is almost completely unregulated.
The child's surname, for example, does not have to be either the father's surname or the mother's surname. Many children, for example, have surnames that are hyphenations of their father's surname and their mother's surname, even if the parents did not change their names upon marriage.
The tradition in Iberia and much of Latin America is for children to take both their father's surname and their mother's surname in a double name surname which is often not hyphenated even though both parts of the name are part of the full surname. The U.S. allows parents to continue to follow this custom as a matter of law, although some governmental and private bureaucratic systems for dealing with people's names in computerized databases don't allow for these kinds of entries and also often disallow hyphens as characters in the name database entries.
The process for naming a child when two parents are designated and they can't agree varies from state to state. Usually, a court would decide, or an unmarried mother would decide.
Also, some states have the common law rule that your legal name includes any name by which you are commonly called and acknowledge as your own, even without a formal bureaucratic change on a birth certificate or other governmental legal process. Every state, however, also, at least, has a bureaucratic or court process by which a person's name can be changed.
3- Is it a crime to have sex with another woman while you have
In a large majority of U.S. jurisdictions, adultery is no longer a crime (assuming the sexual act is consensual and not incestuous) including 33 U.S. states, the District of Columbia, and the several U.S. territories. "States which have decriminalised adultery in recent years include West Virginia (2010), Colorado (2013), New Hampshire (2014), Massachusetts (2018), and Utah (2019)."
Adultery is rarely enforced criminally in the 17 states, as of 2021, that still do have adultery laws on the books. This is, in part, due to doubts about the constitutionality of these crimes under federal and state constitutions, in part, due to changing norms, and in part, due to the limited benefit of a misdemeanor or felony prosecution to all persons involved (and the state) in such cases.
The birth of a child who is conceived with a father other than the mother's husband while the mother is married is not automatically conclusive proof of the crime of adultery under either state law or under the U.S. Code of Military Justice.
For example, under South Carolina law adultery involves either "the living together and carnal intercourse with each other" or, if those involved do not live together "habitual carnal intercourse with each other" which is more difficult to prove. Similarly, in Florida, the crime is "Living in open adultery".
Also, in the case of a prosecution of an unmarried man, knowledge that the woman is married would typically be an element of the crime of adultery, and it is similarly never a crime to be raped in the U.S., even if you are married.
In 13 of the states where adultery is still a crime (Arizona, Alabama, Florida, Georgia, Illinois, Kansas, Maryland, Minnesota, Mississippi, New York, North Dakota, South Carolina, and Virginia), it is a petty offense (the maximum punishment in Maryland is a $10 fine), or is a misdemeanor. But it continues to be a felony in four states (Idaho, Oklahoma, Michigan, and Wisconsin) and is punishable most severely among those states in Michigan who someone convicted of adultery faces up to four years in prison.
It is a crime that is actively enforced for active duty members of the U.S. military under the U.S. Code of Military Justice.
In the U.S. military, adultery is a potential court-martial offense,
falling under the General article (Art. 134). The Manual for
Courts-Martial defines (para. 99) "Extramarital sexual conduct" as
being: "Elements.(1) That the accused wrongfully engaged in
extramarital conduct as described in subparagraph c.(2) with a certain
person; (2) That, at the time, the accused knew that the accused or
the other person was married to someone else; and (3) That, under the
circumstances, the conduct of the accused was either: (i) to the
prejudice of good order and discipline in the armed forces; (ii) was
of a nature to bring discredit upon the armed forces; or (iii) to the
prejudice of good order and discipline in the armed forces and of a
nature to bring discredit upon the armed forces". . . . The law
on adultery was revised in 2019 in order to include same-sex
encounters in the offense.
Neither the U.S. military, nor any U.S. state, has penalties for adultery that differ depending upon whether or not the adultery results in the birth of child.
The U.S. Supreme Court has not ruled on the constitutionality of adultery crimes since its ruling in Lawrence v. Texas, 539 U.S. 558 (2003) which decriminalized any kind of sex between consenting unmarried adults that does not constitute prostitution, as a matter of constitutional law (including sex between married adults with each other).
4- Is it crime to father a child outside of marriage?
No (assuming that rape or incest is not an issue).
Laws prohibiting unmarried consenting adults (or consenting adults who are married to each other) from having sex (including anal sex) or children (in non-incestuous relationships and not in violation of a position of trust) are unconstitutional in the United States pursuant to Lawrence v. Texas, 539 U.S. 558 (2003).
So, even in states where there are laws prohibiting people who aren't married from having sex with each other or living with each other on the books (the former were called "fornication" laws, and the latter were laws prohibiting "cohabitation" of unmarried couples), those laws are unconstitutional (and in the case of laws barring cohabitation are also prohibited by federal fair housing statutes).
Unconstitutional fornication laws, which effectively make all forms of sex outside marriage illegal remain in the law books without being formally repealed in six states: Idaho, Illinois, Massachusetts, Mississippi, South Carolina and Utah.
Related Non-Criminal Matters
Civil Lawsuits Involving Adultery Or Rape
In a handful of U.S. states a husband can bring a lawsuit for money damages (called alienation of affections or "criminal conversation" despite the fact that it is a civil lawsuit) against someone who has sex with his wife. The vast majority of U.S. states have abolished such lawsuits, however.
In fiscal years 2000–2007, there were an average of 230 alienation of affections filings in North Carolina per year — a bit over 0.5% of the number of all divorces. The tort is also recognized in Hawaii, Illinois, Mississippi, New Mexico, South Dakota, and Utah, but it is frequently litigated only in North Carolina and in Mississippi.
Many of the states where the tort is not commonly used impose restrictions on it that make it less attractive. In Illinois, Hawaii and New Mexico, these limitations make it exceedingly difficult to prevail in an alienation of affections case and recover substantial monetary damages. The standard of proof is lower in Utah and South Dakota, which continue to have actively litigated alienation of affections suits, although not used as often in these states as in North Carolina and Mississippi, for reasons that are presumably unrelated to the relevant legal standards that apply to these lawsuits.
Also, in a rape case, a rape victim can bring a civil lawsuit for money damages against the rapist for assault and battery in most cases, and in those cases, many states allow a spouse of a rape victim to bring a parallel lawsuit against the person who committed the assault that is called a lawsuit for loss of consortium. The right to sue for loss of consortium is not specific to rape cases; it applies in all lawsuits for personal injury where it is available.
Relevance To Divorce Cases
In most U.S. state, divorce is granted without regard to marital fault and adultery is not considered in alimony awards or property divisions.
While every U.S. state has some form of no fault divorce, and number of U.S. states also have fault based divorce, and a number of U.S. states, however, allow consideration of adultery as a form of marital fault in divorce proceedings, including how much, if any alimony is awarded and in property divisions.
No state would consider being raped an act of marital fault, however, even if this caused a wife to give birth to a child whose biological father was not her husband, and even if the husband wanted her to have an abortion.
Relevance To Children's Rights
Adultery generally can be considered in child custody cases only to the extent that it impacts paternity or it was something that a child perceived personally, because every U.S. jurisdiction uses a "best interests of the child" standard for making child custody decisions, and because child support is subject to indirect federal regulation by conditioning federal welfare funding to states based upon following federal regulations related to child support.
Parents do not automatically have priority over non-parents in proceedings to determine child custody although there is a strong presumption in favor of parents in such cases, even when termination of parental rights is not at issue. A legally recognized parent has a right to only minimal, roughly annual, sometimes supervised, visitation with a child.
Parents do not have the full rights of a legal guardian with respect to their children and the rights that they do have with respect to their children are often not spelled out affirmatively in detail in statutes or case law.
The U.S. Supreme Court has held that it is illegal for a U.S. law (including a state or local law) to treat a child born out of wedlock whose paternity is established differently from a child born to married parents. For example, the biological father of a child conceived in an act of prostitution with an unmarried mother can be legally established as the father of the child by the mother in every state. Prior to these rulings of the U.S. Supreme Court, many states did not allow children born out of wedlock to inherit from the intestate estates of their fathers, and did not create a right to child support connected to children born out of wedlock.
Also, it was historically much harder for a child with a U.S. citizen father and a non-U.S. citizen mother born outside the United States to claim citizenship than it is today, although this still requires a legal process to establish citizenship that is more difficult than for other children when paternity was not promptly legally established between that father and that child.
But the courts have permitted fairly onerous burdens to be placed on a child or a father seeking to establish paternity in either an out of wedlock birth or a birth to a married woman when the biological father is not her husband.