"Rape is wrong" is very subjective and definition dependent. For example, the idea that's it possible to rape one's spouse is quite a new concept, and certainly not a worldwide one. Also, various versions of rape depend on age of consent, which ranges between the Vatican (age 12) and Tunisia (age 20).
So what? Moral rightness and wrongness doesn't depend on when, it has to do with justification.
It may also be descriptively true that rape is not a moral wrong universally, but descriptive truths are not the point at issue. The point at issue is the justification for normative judgements.
Rape being wrong may certainly turn on consent, but age of consent isn't what we rely on to characterize rape. The fact that rape sexually violates a person's physical integrity is what constitues rape.
In any event, you're still missing the point at issue: is epistemic relativism and similar moral relativisms based on the assumption that acceptance alone dictates the rationality of a standard a justified assumption?