Do you think that's it?

Yes actually 99% of movies outside of porn will not show the vagina apart from public hairs, but...

Idunno if that's it.

It's not like 'well no one's interested in seeing a penis if it isn't erect, so why bother...'

If not now, then I know there at least used to be laws/rules that you absolutely CAN NOT show an erect penis in films, not in U.S. movies anyway.

Apart from non-porn, and this includes softcore--I can only think of two cases of this: Lie With Me--a Canadian film--and Catherine Breiullet's Anatomy of Hell.

I don't know, I'd imagine that sometimes the censors balk at showing erect nipples too.

I mean I won't suggest that I know exactly what kind of movie nudity turns on the average woman. But I also think most of the people making movies largely don't give a damn.