If u had a girlfriend, and a twin brother,(and u and him were real damn close), and your twin brother got in a car accident, and was going to die in 24 hours, and your twin brother ask u if he could fuck your girlfriend before he died, would u let him?
I'm pretty positive it would be up to her. Unless you were planning on decieving her. Then I suggest you break up with her and explain to her that she deserves better than you because you don't care if she has the freedom to choose her sexual partners or not.