For young kids, it makes you cool to label something as "gay". I think it asserts their masculinity or something.
It doesn't really bother me that much anymore. If I started posting to those threads and informed them that calling things "gay" isn't very nice, I'd just get flamed back to the stone age. I'm sure they'll grow out of it at some point.
By the way, have you ever seen this one? I get a chuckle everytime it pops up:


