I know this is probably just a joke, but the incest taboo is naturally programmed into our brains. Gay people fucked each other while it was still a societal taboo, something being taboo or illegal is not enough to stop people from doing it when nobody is looking. If it was just society prohibiting it, incest would probably be as common as underage drinking. But it's not, because we're naturally hardwired to find it gross.

Alright so you know that sweet grass you and your buddies like to eat? If you throw some of it in a bowl with some water and put it over the fire, you can make this stuff we call "booze" and if you dump it on your wounds, you might not die from the infections evil spirits.