Guy loses girlfriend on dare or game at a party

[scubajoe1234]

Trying to remember a story about a guy who brings his girlfriend to a party with either older co-workers or something. They end up joining in on a game or a dare or some kind of challenge, and one of the other guys ends up "winning" the girlfriend. The main guy ends up sleeping in a bathtub listening to his girlfriend get pounded by someone else, and one of the other girls at the party lets him jerk off while watching her strip in the bathroom.

Any ideas?