Oh gods! So close, so, so close. You are right, it can be compared to shonen-ai, but that's because, yaoi is the american term for it.
Wikipedia has the information you would need about yaoi and the way it came to be. But in short, it says:
Shock and awe: yaoi is BL! Yes, Boy's love and yaoi is one in the same.
Alright, one more quote:
See, yaoi is just BL under a different name.
Now that is cleared up, I enjoy yaoi. I prefer the movies such as Level C and Legend of the Blue Wolves (has rape and sex in it). I also enjoy the cutsie anime Gravitation. I like yaoi, though not the terms. I do use them, but that's only because of the fact that I also write Fanfiction since it's such a great way to practice my writing without worrying about characters and what not.
If you take a bit of time and look for the right stuff, it can be enjoyable and a very wonderful watch, read or listen (depending on what you're doing.)