It's a cousin of Rule 34. If a thing exists long enough to get noticed, there will eventually be people who like to RP as characters from it. Some of them sexually.

Rule 35 is that if there's no Rule 34 content for something, well never mind, there is now. I feel like that's also true with roleplayers - people want to RP something *because* nobody else is. There's always someone in the 501st who's like "wait, so that means Watto isn't taken?"