I will use Washington law to lay the landscape for a rape charge. First degree rape is sexual intercourse with another person by forcible compulsion, either with kidnapping, (apparent) threat of a deadly weapon, beating the victim, or feloniously entering the building of vehicle where the victim is. This is not the case under discussion. What does potentially apply is second degree rape, which is sexual intercourse by other types of forcible compulsion, or (b) When the victim is incapable of consent by reason of being physically helpless or mentally incapacitated, or various similar descriptions of diminished capacity (a developmental disability plus perpetrator supervisory authority, etc). Subparagraph (b) is crucial here, and we turn to the definitionL
"Mental incapacity" is that condition existing at the time of the
offense which prevents a person from understanding the nature or
consequences of the act of sexual intercourse whether that condition
is produced by illness, defect, the influence of a substance or from
some other cause.
Also to be clear on "forcible compulsion",
"Forcible compulsion" means physical force which overcomes resistance,
or a threat, express or implied, that places a person in fear of death
or physical injury to herself or himself or another person, or in fear
that she or he or another person will be kidnapped
In the case of intercourse with an intoxicated person, the government must prove that the victim's condition prevented them from understanding the nature or consequences of intercourse.
Now compare the law in Minnesota, otherwise analogous, but with a different definition of "mentally incapacitated" (subd. 7):
"Mentally incapacitated" means:
(1) that a person under the influence of alcohol, a narcotic,
anesthetic, or any other substance, administered to that person
without the person's agreement, lacks the judgment to give a reasoned
consent to sexual contact or sexual penetration; or
(2) that a person is under the influence of any substance or
substances to a degree that renders them incapable of consenting or
incapable of appreciating, understanding, or controlling the person's
(italics indicate the differences of interest). The second clause in the Minnesota definition states the same idea as the Washington definition, but also adds "inability to control" as a form of incapacity. The first clause states a different standard of inability, lowered from "incapable of deciding" to "lacking judgment", but only when the condition arises without the person's consent.
The point here is that prosecution and conviction do not just depend on a generic and universal concept of "consent", it very much depends on the exact words selected by the legislature in establishing these laws. Minnesota's legislature made one set of choices, Washington's made another.
The Washington legislature made another choice when it came to intoxication: per RCW 9a.16.090,
No act committed by a person while in a state of voluntary
intoxication shall be deemed less criminal by reason of his or her
condition, but whenever the actual existence of any particular mental
state is a necessary element to constitute a particular species or
degree of crime, the fact of his or her intoxication may be taken into
consideration in determining such mental state.
So an voluntarily drunk person cannot defend themselves saying "I was drunk, I didn't know what I was doing", but intoxication can be relevant to the important question of "knowledge" as an element of culpability. This is a principle of law specifically set by the legislature.
Prosecutors don't decide based on a hypothetical "how drunk" scale, instead they compare the facts with what the applicable law says. There is no "knowingly" requirement for prosecution for rape, therefore one cannot argue that because of intoxication the accused "lacked knowledge" of their act owing to intoxication. Being intoxicated according to the .08 driving standard does not (generally) render a person incapable of understanding "intercourse".