Yeah, so I think this was a good thing to have happen to AJ, but I don't share in the "Ha! In your face! Yes!" mentality. AJ was probably hurt, and that's never something to be happy about. Again, it was a good thing in this case, and hopefully he'll learn something from it, but that's a separate issue to me. It was a necessary evil, as far as I'm concerned. (Not that I think the act on the side of Robin/Rodney was evil, of course—not at all.)
I wouldn't even say there's anything wrong with AJ wanting to "just get his rocks off" and have casual sex with lots of people. He just has to be honest and upfront about it. It's not enough to never say he wants a fullscale relationship (which actually he did more or less say implicitly, and he has used words like "love" and such, which makes things worse); he has to explicitly say that that is not what he is looking for. The culture he lives in is such that there is often an implicit suggestion of a relationship following from sex, so he has to negate that explicitly. If he does that, as far as I'm concerned, he can have as much casual sex as he wants. Although I do wish he'd be safe about it.