The book I'm currently reading, Vellum, has two (or more) homosexual characters who engage in sex in the text. That's why I was thinking about this.
I found the sexual content of
Vellum striking, partly because it was uncommonly graphic for SF, but mostly because it was uncommonly... exuberant. For one of the most important characters, sex is his animating force. For him and one of the other characters, lust is the defining aspect of their relationship. To shy away from the sex or to keep it decorously off camera would be to fail to portray these two characters accurately (and there's a structural reason why the characters are as they are that I won't discuss further). The fact that these characters are male in their various incarnations ineluctably leads to a lot of gay sex.
By the end of
Ink, I was more inclined to describe the sex as "relentless". There's a lot of it compared to most other fiction, within the genre or without. But then there's a lot about
The Book Of All Hours that's relentless. Similar stories are played out repeatedly in different folds of the vellum (I promise that this statement makes sense in context) so it's inevitable that the same characters are repeatedly shown having sex. When everything in the story tends towards excess, it would be unfair to pick out the gay sex as gratuitous.
It won't surprise me if in 25 years' time this topic is barely even remarked on any more. I'm sure different authors will differ in the quantity and explicitness of their gay sex scenes as much as they differ in their straight sex scenes (or technobabble, or depth of character development, or pacing, or any other element of their style), but there will probably be enough explicit gay sex that it won't seem particularly surprising.
Just now I can think of one obvious reason why "serious" SF authors might be writing more explicit scenes of gay sex (or cross-species sex in one of the China Mieville examples mentioned above): frequent descriptions of heterosexual sex described from a male character's POV could be seen as pandering to the traditional SF demographic of nerdy teenage (or never-grew-up) males who aren't getting enough. You can avoid this by not writing about sex at all, but it's very limiting for an author to ignore such an important part of what it means to be alive. Or you can write scenes that are less likely to be seen as trying to boost sales with cheap titillation. (Not that I believe that Hal Duncan or China Mieville would let such cynical motives drive their writing.)