What FSF said.
I would only add to think of your story and intent. And what Pixar is said to live by: story, story, story.
So, if you're writing erotica, like
Red Shoe Diaries, or something like that, then the love scenes really ought to be fairly explicit and, well, erotic. Still, as FSF says, probably leave that up to the director and the actors, etc.
But let's say that the way in which the characters have sex is significant to the development of the story or the characters (and so, the story), then you might want to describe it. For example. In
Young Adam, the sex is decidedly and deliberately unromantic. I haven't read the
Young Adam Screenplay. But the nature of the sex is important to the characters' interpersonal relationships and so to the story. So if I were writing that, I would probably find some way of conveying that it should be unromantic. It's an interesting question. Did the writer of
Young Adam devise that, or did the director and actors? Ahhhh, so it was written and directed by the same
man. That may be telling, huh?
But imagine a very different story in which the sex should be very passionate or loving or tender etc. If it's important to the story, I would think that it's up to the writer to at least suggest that. Anyway, I can easily imagine how a screenplay could benefit from the writer at least describing what tone is appropriate.