Science fiction of the 70's and 80's was rife with gratuitous, explicit sex that did nothing to move the plot forward, and it wasn't "erotica." It was there purely for titillation, and as a young adult, I sought out those books. I'm thinking, for example, of the Richard Blade series, among many others, that featured a man who traveled across parallel Earths, having sci-fi adventures interspersed with multiple gratuitous sex scenes. They were marketed as science fiction, and that's the section you found them in in the libraries.
Would people avoid reading Edgar Rice Burroughs if his Barsoom novels were sprinkled with dozens of pages of explicit sex? I think not, because despite the sex, they would still be great science fantasy stories.
The western genre as well contains plenty of series featuring highly explicit sex scenes: Longarm, Slocum, Gunsmith, etc. I doubt anyone would consider these erotica. They're clearly westerns. Walk into any library and you'll find them right alongside Louis L'Amour. And they're not in the Amazonian dungeon, despite having very pornographic sex scenes and racy covers.
So put all the nudity and sex you want in a non-romantic science fiction series. Just because it's not a romance doesn't mean it can't have sex in it. I think the only question is whether you're morally comfortable putting it in your books and thereby guiding your reader's thoughts into such territory.