I agree with the general idea that yaoi
can sometimes be like shoujo and/or josei, but not always.
I said this in a different thread somewhere, but I think in general, those genres (shoujo, josei, and yaoi) typically focus on the emotional bonds between people more than just the sex, which I find to be a problem with most hentai (but not all). It's the emotional bonds that tend to attract female readers.
Why yaoi over shoujo? Some of the better yaoi isn't like shoujo in that you get a different type of relationship/interaction than you get from shoujo, but the overall themes and art are still similar. And sometimes, you just want to see two cute guys in a shoujo-ish story/setting instead of a guy and a girl.
I agree with Odette in that as a straight girl, it's easier to empathize with characters who are attracted to men.
________________
If you've ever thought or said "Nice Guys finish last" and
really meant it, then you should probably
read this LJ post by DivaLion. It's incredibly insightful whether you're male or female.
From a bumper sticker I like:
"If you're gonna ride my ass, at least pull my hair."