In general, the audience gets uncomfortable if certain aspects of a film are ambiguous. For example, if the audience is shown a politically-charged theme (for example a teenaged girl about to get an abortion), the moral framing must be defined: is this an acceptable thing or not?
Even if the audience disagrees with the political stance of the director, they will be much more comfortable knowing they disagree than not knowing whether they agree or not.
Genre is a kind of contract between the audience and the director. The audience, adverse to a certain kind of uncertainty, needs a bit of definition of the type of thing that is possible in the universe they are observing. If the narrative changes genre in the middle, this can make the audience cranky. Even if the change is clearly intentional, critics get upset; audiences feel misled.
Two great examples: “From Dusk Till Dawn” and “Death Proof,” both joint Quentin Tarantino / Robert Rodriguez productions. In both movies, the story starts very firmly in one genre, then abruptly changes genre. You can read reviews online to see what people thought of that… I liked them, anyway.
So that said, the basic issue with Horror and nudity is one of genre. As above, you as the director need to define the movie as a horror movie very early in the piece. But, Horror is a genre, like Action, that requires a bit of crazy action right at the beginning.
What makes Horror fragile is its dependency on “suspense,” which is in turn contingent on the audience’s anticipation of an outcome adverse to the characters. The problem here is, in order for the audience to feel suspense, they must be emotionally engaged in the welfare of the character who is about to be a victim. And this must happen at the very beginning of the movie.
So: how do you make an audience member care, on some level, about what happens to the character they’ve just met? What will be the agent that “involves” the audience on some emotional level? You only have a few seconds; the movie can’t get old while we wait for the first killing.
For men, the “agent of involvement” turns out to be… hot girls. With bare boobs. “Oh no, don’t kill the girl with the killer rack!”
For women, it’s a little more complicated. Just as in advertising, when you show a sexy woman, the men want to sleep with her, and the women want to BE her. The female viewer identifies with the beautiful woman.
But not too beautiful! If the woman on the screen is too out of her league, the female viewer cannot identify with her and gets annoyed. The woman on the screen must be in some way “at the same level” as the female viewer.
So, more slutty, and you get the attention of the male audience. But too slutty, and you risk blowing off the female audience. Also the woman is marginally less desirable to the male audience if she is TOO “gettable.”