I've been the other guy. Personally, I'm reluctant on account of the fact that I want to know I'm sleeping with more people than she is. I'm only kidding. I don't usually like the idea of being with someone I know is sleeping with someone else since obviously your risk of catching something goes up since she's cheating on her significant other with you and very well may do it often. Also, who knows what the **** her boyfriend is up to. Nevertheless, you only live once. It's fun to do stupid things every now and then.