The most convincing argument I've heard against yaoi so far is, "How are they gay?"

I don't know about other yaoi/shonen-ai fans, but personally, I don't expect them to be gay. Fanart is not canon. Yaoi is not canon. Unless you get one of those yaoi manga, which aren't really good anyway. In fact, I don't like yaoi manga. Out of all aspects of yaoi, the most fun part - or at least second most fun - is twisting the story. With the given information, distort the story so that two given male characters have more than platonic feelings for each other.

That argument is null and void, because it's something that we, or at least I, acknowledge. They're probably not gay. In fact, more than likely, they're straight. But there's no fun in shipping a canon pairing. If you were shipping a good, semi-crack pairing, and it suddenly became canon, wouldn't it seem less exciting?

Yeah, I like non-canon pairings.