It doesn't HAVE to be, but, asides from RL considerations of inclusion, can be an enrichment for a game world. If I create a world, not every place is the same. Maybe it's ok and even the norm for the Deep Dwarfs to live in homo relationships, male couples raising the boys and females the girls. Yet the Hill Dwarfs only think it fine if both of the couple is infertile, or the Silver Elves don't allow it because there are so many of them left and they need couples to produce children. And maybe the Lowland Halflings don't even live as couples of any kind, but in married groups of 5 or 6, and male and female are just concepts to go by if they want offspring.
There was a set of captioned gifs I saw once from something (I think Sleepy Hollow maybe?) where someone who has been out of touch with our society for a very long time indicates two people and says "is that kind of thing acceptable now?", and gets a nice little lecture about same-sex couples and acceptance and tolerance, to which he responds "I meant men wearing hats indoors."