1. A gayboy has been bumming, and he has got a wet mark at the back of his trousers/shorts/etc. where he has been bummed.
2. A gayboy bums another gayboy. He gets wet FROM the back, because it is the gayboy he was bumming's back that got him wet. He has a mark like he has pissed himself for ages.
2. Max, have you pissed yourself, or are you wet from the back?