The Japanese psychology textbook answer to this is that yaoi allows younger women to gawk at a male initiated sexual or romantic relationship, but one that is less intimidating to their own female bodies (because the female body is absent).
I dunno if that's true. I'm not a chick or Japanese. I have always found it interesting that both the creators and chief consumers of yaoi are overwhelmingly straight females though. There may be some truth to it. It also seems to have carried over to the west because...
My child...
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"I've... SEEN things you people wouldn't believe."
Yaoi-Con was (somewhat counterintuitively) pussy city. Albeit perhaps not the "choice pussy." The 85% figure quoted in the Yaoi-Con Wikipedia article is probably true. It might even be too conservative. A legion of straight girls "squeeeing" over their favorite bishounen. It wasn't gay dudes in attendance like one might expect. A fact made more shocking by the fact it was held up in the Bay Area.
Erica Friedman also did a YuriCon out in New Jersey. She's an old lesbian that sometimes hangs out with ANN (she was on their Kill La Kill podcast show before Zach offed himself). I think that one appealed more to lesbos. It was also way smaller.
The "interesting" point to make here is that if you're a straight dude looking to get laid to weeb girls, and your standards aren't sky high, you actually go to the yaoi con and cosplay as a bishi character. Not the yuri con, or even the generic anime con. It seems weird, but I had a friend who cosplayed as some bishi manga character, and he ended up having a threesome with two chicks in sailor fuku that weekend at the convention hotel (one of whom was 7/10 hot). Not traps. Not lying. Almost zero effort outside of the costume construction, and bishi characters have stupidly simple costume construction (open button up shirt with bare waxed chest, longish hair, et cetera).
It was simultaneously amazing and horrifying. Probably more horrifying, but the story is fun to tell.