Evidently protection against sexual harassment dates to Article 7 of the Civil Rights Act, which makes it "unlawful" for employers to discriminate on the basis of sex. Is there something in the language of the act that makes sexual harassment a civil rather than a criminal offense? Was that up to the lawmakers to decide, or is that just something that evolved out of the ways the courts and prosecutors responded to sexual harassment claims? And who decides such things in general?
Is there something in the language of the act that makes sexual harassment a civil rather than a criminal offence?
Laws that create criminal offenses have to have language to the effect of "violation of this law is punishable by up to X years of imprisonment or a fine of up to $Y", or "violation of this law is a Class Z felony."
Sometimes it is not entirely clear if violation of a law can form a basis of a private civil lawsuit, or if it can only be enforced by government officials, from the language of the statute alone. When it is unclear the courts have to resolve that ambiguity.
In rare instances, it may be clear that some parts of a statute have criminal penalties, but due to unclear wording and punctuation in the statute, it is hard to tell precisely which parts of the statute these criminal penalties apply to, and in those cases, courts also have to resolve that ambiguity.
There is also some conduct that it is constitutional to punish with a civil penalty, but not as a crime that can result in incarceration. For example, it is unconstitutional in most states to incarcerate someone for failing to pay a debt, but there can be a civil penalty for failing to pay a debt. Courts decide if these constitutional limitations are violated.
Similarly, while Congress can enact both crimes and civil penalties, there are some governmental bodies, like school districts or water boards, that have the power to enact certain civil penalties, but do not have the authority to create new crimes.
Was that up to the lawmakers to decide, or is that just something that evolved out of the ways the courts and prosecutors responded to sexual harassment claims? And who decides such things in general?
Generally, this is decided by lawmakers. Obviously, however, anyone can lobby legislators to take one position or another.
Also, the fact that something has a civil penalty does not necessarily mean that prosecutors aren't the people who enforce the law. Sometimes violations of the law prosecuted by prosecutors have civil rather than criminal penalties. For example, many tax law violations are prosecuted by government lawyers with civil penalties, but only a small minority of tax law violations are prosecuted criminally.
Further, it isn't uncommon for a type of offense, like securities fraud, to have both civil penalties and criminal charges available as remedies that can be enforced by prosecutors. And, when that happens, prosecutors get to decide which tool to use.
For example, even if exactly the same conduct could be prosecuted with either a civil penalty or a criminal charge, prosecutors might prefer a civil penalty because the burden of proof is much lower, the 5th Amendment protection against self-incrimination does not apply (you can refuse to testify but that fact can be used against you in a civil penalty case), and a defendant in a civil penalty case doesn't have a right to a lawyer at government expense. Also, enforcing a civil penalty generates net revenue for the government most of the time, while criminal punishments normally cost the government more money to carry out than any revenue the government may receive from the person found guilty for fines and court costs.
On the other hand, trying to enforce a significant enough civil penalty to discourage misconduct against someone who has no money or property may be a futile effort, while criminal sanctions could discourage misconduct from other similarly situated people in the future.