"Maybe the reason the penis is so mysterious is because it's an image that girls are sequestered from on cultural reflex?"

This right here. I'd say this definitely plays into at least the fascination aspect of yaoi. Perhaps not why a homosexual relationship is more functional than a heterosexual one... but it at least explains WHY so many slash fans are women.

I agree that a LOT of it is because we're taught basically from birth that nudity is lewd and the human body is something to be hidden away. I'm not saying we all need to strip down to our skin and run around ... because that would be uncomfortable for other reasons. But we're conditioned to view the body, particularly that of the opposite sex, as a taboo. At least part of this stigma is due to Victorian ideals of propriety. That era, oddly enough, is responsible for a LOT of lingering social misbeliefs. *has been doing research*

Anyway, another part of the fascination may be because women don't HAVE penii. It's something completely foreign to us; a mystery, as you said. And who doesn't want to explore a good mystery? *leers*