Yaoi Fanboy; as rare as they may be, are members of the male sex who enjoy Yaoi, I.E Boy on Boy/Homosexual relationships. They are usually fans of the same things as the Yaoi Fangirls (See Yaoi Fangirl for meaning) and have usually become friends with at least one of them. A Yaoi Fanboy can be a straight, Bi-sexual or a Homosexual male, they are not limited to one preference.
Person 1: OMG, you like Yaoi?! but you're a guy.
Yaoi Fanboy: Hey, as long as (Insert favorite pairing) get it on, I'm happy.
Yes, they do exist, you just aren't looking hard enough, or you think they're a girl..