Well, the master of high Fantasy, JRR Tolkien, had no sex in LoTR at all. Not even implied. And the early masters of space opera Sci Fi (Heinlein etc.) were often writing for younger readers, at least early on. And in the middle part of the 20th century, there was a general prudishness about sex (not to mention a near invisibility of women in most of the stories, so...) Maybe that set the tone for the genres.
But some of the more modern writers in the genre have relationships in their stories that are clearly sexual and where sex is strongly implied, even shown in some cases (Lynn Flewelling, Mercedes Lackey, Robin Hobb, Brent Weeks, George RR Martin, Glenda Larke, even Anne McCaffrey are some of the ones I've read that do. I haven't read as much sci fi lately, though I remember some of CJ Cherryh's books had strongly implied sex in them at least), though it usually isn't super graphic. Sometimes the details sort of stop when the couple hops in the sack all hot and bothered and the author leaves the culmination of the act to the reader's imagination. I always assumed it was a publisher thing.
[edit] And I agree with the previous poster about ASOIF. Lots of sexual tension and the occasional use of graphic language, but the actual act itself is described in a fairly perfunctory manner.
But that's just a guess, and maybe it's been changing. Maybe it's the authors' preferences. I don't mind sex being a bit more graphic than is traditional, as long as it's written halfway well, but I don't mind the details being left to the imagination either. I like there to be romantic subplots and such in stories if they are done well. It's human nature to be attracted to one another and to fall in love, even when you are trying to save the world. Romantic and sexual tension adds an extra dimension to stories, raises the stakes for the protagonists and makes the characters more believable and relatable, imo.