Well, science fiction itself is futuristic and sex is a popular topic. It's not clear that futuristic portrayals of sex in SF need to be explained, any more than futuristic portrayals of eating/food, travel, politics or society.
Or portrayals of sex in historical fiction, epic fantasy, classical mythology, spy novels...
It is interesting, though, to note that while this is true, humans are very complicated creatures insufficiently specified by our genes and are readily able to tie sex mentally with all kinds of things given the proper circumstances.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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