Further my experience when I finally left XP wasn't so much lack of belief as "not wanting to take sides".
I experienced very much the same thing when I left mine. But I think this can be applied to rape too.
Stranger rape, which is statistically rare, is treated with near universal condemnation and people do generally believe the victim. That's not the case with most rapes, which usually involve people who know each other or who at the very least spent some time in friendly chat before the rape took place (e.g. date rape).
Rapes do not take place in a vacuum. People know both victim and rapist, and often they know both and mix with both. It makes things too uncomfortable for people and they don't want to take sides. Far easier to minimalise it and make the victim partly responsible (she was drunk) etc. "They're both responsible" removes blame and allows people to carry on their own lives without taking sides and making a stand. Most of this is done subconsciously of course. It ends up making rape condoned even if people say they don't agree with it, much the same as DV. Actions are not mirroring words.
The trouble is, if you have this prevailing attitude in society, it is undoubtedly going to be reflected in the legal system. The only way to get the legal system changed, where women's characters and sexual history are not held up for such detailed scrutiny in a way that just doesn't happen with other crimes, is to make rape socially unacceptable full stop. That means people have to stand up and be counted and condemn rape, under all circumstances, with no excuses and however uncomfortable it makes us feel.