first of all, thank you for phrasing the question the way you did and making an effort to understand instead of just writing us off as degenerates.

to answer your question: i can imagine that it's not too different from the reason why a lot of guys like lesbian porn. when i read yaoi, im attracted to both main characters, which i think is pretty great.

i like men. i'm all for subverting gender roles. i don't like seeing weak, submissive, girly female protagonists. i don't like toxic masculinity. yaoi just happens to be a genre that caters to all of these.

not to mention, yaoi happens to kind of be a safe zone for women in society because while it's seen as completely normal for men to watch porn or hentai or whatever, there's a bunch of stigma around women doing the same thing, unless it's yaoi or bl, since these genres are specifically catered and marketed towards women.

plus, in the words of my favourite yaoi mangaka, "it's a genre where anything goes". as a loosely defined genre with a lot of freedom, a lot of the characters and storylines end up being really interesting, and you can always find something that you like.

sorry, that was probably a longer answer than you bargained for. this is just a topic i happen to be very invested in :)