Why do men like girls going down on other girls?

This may be a seriously stupid question to ask, but I’m just curious. I have a boyfriend that I’ve been dating for over a year now, and I’m pretty sure one of his kinks is girls being sexual with other girls. He has told me all of his kinks but left out that one, and I’m pretty sure he’s just too shy to say which I’m completely fine with. But in some shows I watch, it seems like that most men are like that? They like seeing lesbians make out or having sex and I was wondering if that’s the nature of men?