At least of the ones I've seen, they're done with some kind of dramatic or thematic push, and there's no real "tittilation" at all. It's like uncomfortable or weird sex, or different or unsettling in some way, and if done right, it's an extremely intimate view into the relationship between the two characters.

Yeah, it's definitely done for attention, too, but don't act like they're just randomly throwing porn on screen and seeing if it sticks. That's Caligula, not these movies.

Sex in movies is always a weird point. If you're not just throwing it in for the audience appeal, you always have to answer a lot of tough questions about why and how you're going to put it in there.