From the episode Sippin and Strippin.
These crazy chicks started to strip down while partying. One of the guys got so horny that he pulled out his big wiener in front of them. Ava Taylor took the bait and started to blow it in front of the party peeps. Serena thought it looked fun so she grabbed a dong of her own and put it in her mouth. A foursome evolved which you can see below. Or you can go straight to the full scene.
