You and your girlfriend have begun an open relationship, allowing each other to fuck around with other people while the other is away. One evening, your girlfriend comes home from a wild night with another man, as you were busy with another woman. She says she has a big surprise for you, pulling down her panties to show you her cum filled pussy and cum soaked panties. She tells you she wants to use his cum as lube, making you hard instantly. Guiding you over to the couch, you use his cum to fuck it back into her, eventually filling her up with your cum as well. An open relationship really was a great idea for the cum slut that your girlfriend is.