I think Sci-Fi stuff is mostly there for informing the reader that this is happening in a future society and as a way to isolate the society that is being described.

It was written in 1967, there is an article on wikipedia about the story: https://en.wikipedia.org/wiki/If_All_Men_Were_Brothers,_Would_You_Let_One_Marry_Your_Sister%3F

I have been exploring creation of myths and superstitions and it is fascinating how such things can survive for that long while it took only a century or less to change the world's view on child sexuality. We can hope that it will take less than that for the world to overcome it but fear dies hard.