My girlfriend is giving me my ual fantasy..but...?

Ok...me and my girlfriend are both in college.For some reason, my fantasy has always been to see my girlfriend have with another man. I do satisfy her so it isn't inadequacy, but I just want to see it. I talked it about it with her and she was not wanting to do it for a while. Until, however, I brought up the idea of a foursome with a open-relationship gay couple whom are friends that I wouldn't mind experimenting with. She actually agreed! The idea is for all three of us males to bang her back to back..me going last. They will probably do things to me too but I don't mind. My question is....my gf says she doesn't WANT to do this, but she is willing to give me my fantasy just ONCE. She is nervous that I will think she is a slut and will want to leave her, which I wont. I am nervous she will hate me for it and leave me but she says she won't. So do you think this is something to try or leave it alone? Oh..and also..this idea turns me on very much..however, the idea seems repulsing just after I masturbate. For like one minute I feel so bad for wanting it. But then I get turned on again and want it again...so whatcha think?