I think it's both genetic and cultural. Both men and women (yes, even women) are stimulated visually, but it's naturally stronger for men and also more culturally accepted. So I don't think there is a black and white answer one way or another. Straight guys are naturally wired to be offended at the site of a naked guy, which might also explain why there are less instances of male nudity (a guy walks around naked: "dude, put some clothes on!"). On the otherhand, American men tend to be more sensitive to being accused of being gay. I don't think European men care as much so male nudity in a movie is not as taboo. So again, a combination of both genetics and culture. The U.S. has stronger nudity laws than some other places (cultural), but it applies to both men and women.