I had to think about this for a moment, as I’ve seen quite a lot of horror posters, and honestly couldn’t think of a single one that could really be mistaken that way. If you were referring to the image I reblogged earlier of the car with teeth, I’m fairly certain that isn’t art from any film poster I’ve ever seen, unless it was used for any alternative foreign posters (and even then, I would have likely stumbled across it while putting together that photo set I made a while back of killer automobiles).
From filing through the posters I have saved, I found 75 faces which appear to be significantly frightened/uneasy:
I found five that look weirdly happy:
Five that look constipated and/or completely fucking baffled:
And, after scraping enough, I did manage to find five that could be mistaken to be in the throes of passion:
But, the thing is, two of them (the first, and the last) are dead bodies, and the second one is from a Cronenberg film that is highly sexually driven, and the look on the face was likely intentional. The third came from a film notorious for a scene in which a giant, alien maggot basically has it’s way with a woman that appears to enjoy it way too much, and the fourth is from an alternative poster for a film that has absolutely nothing to do with what’s pictured on the poster.
While the horror genre is a prime target for cries of sexism and misogyny (which I don’t particularly agree with), this is one point that I must discredit. As a lover of horror poster art, you are far more likely to find faces distorted in screams of fear and terror, rather than pleasure.
You may be thinking of gallery books, which tend to be filled with illustrations from horror/fantasy artists, which often contain the kind of scenes you’ve described.
The way I see it, a lot of those pictures can be construed as “O-faces”.
During (a true, genuine, not faked) orgasm, sometimes the muscles in the face can’t be controlled. Sometimes the faces look like severe pain or fear. So it’s not really that the horror faces look like O-faces, but more that the O-faces look like horror faces.