Nowadays, it's hard to find a horror movie that doesn't include a nude or sex scene. Majority of the time, it doesn't add to the story. It's only purpose was to fulfill fanservice or just a now typical factor of horror. Thinking back on the famous shower scene of Psycho, did audiences blush at the sight of a bareback?
I think sex was purposely incorporated into horror when the slasher sub-genre was born. It was used as a commentary to the AIDS epidemic and sexual liberation of the 70s. Every character that engaged in sexual activity were "punished" with death and the virgin was "rewarded" with life (and perhaps some emotional scars). Examples of this can be found in classic slashers such as Halloween, Friday the 13th, and Nightmare on Elm Street.
Unfortunately, horror movies, whether part of the slasher sub-genre or not, have decided to include these unnecessary and sometimes uncomfortable sexual moments. They have turned an old commentary into a character or necessity. Any movie with an annoying group of teenage characters or stereotypical party going college students, there's always that one couple who possess an abnormally high level of hormones and that's all their personality is. They don't contribute to the main character's development or progress the story. There's no depth to their character just someone whose purpose is to have sex with another character and be killed off.
Here's a quick example of what I mean. You'll probably get tired of me picking apart the notorious Hostel, but there's a lot wrong with that movie and reasons I dislike it so much. For a good chunk of the beginning, the main characters' focus is on drugs and women...not really women, but sex. I remember sitting in the theater and just wondering when the horror elements would pop in in this "softcore porn" of a movie. Yes, it really seemed that way. The guys would go out, drink, obtain some drugs, and hook up with these prostitutes or whatever. It seemed like that was the whole purpose, especially the guy who knew where to get the girls at. For a more visual commentary on unnecessary sex in horror, check out Cabin in the Woods.