A lot of male bands these days do this to show they are all for gays. Fangirls tend to mistake this for REAL kissing/affection and start a slashfic about it. Most bands are not gay and do that to piss homophobes off.
Fall Out Boy, Cobra Starship, The Academy Is..., Panic! At The Disco, Gym Class Heroes, and My Chemical Romance all part-take in sticking it to the homophobes. They only do this for fun, none of them are actually gay. For what all I know, they could be.
Christian: Ha! They're so homo. I bet they even like it. I mean, who does that on stage just to piss off homophobes?
Sierra: ...They're not. They hate homophobes so they kiss and shit. They aren't gay, Christian.
Christian: They ARE homo! Kissing another guy is, like, the definition of homo!