Without understanding, necessarily, what they're on about -- it's just going after differences. More general xenophobia than homophobia, I think. I remember kids trading "gay" and "lesbian" as insults in grade school without exactly knowing what those things meant, or (more importantly) not actually ascribing homosexuality to the target -- it was just something they'd heard and were reusing.

But yes, kids will go after pretty much anything that presents itself, if they feel like tearing another kid down.