No offense but you sound silly.
Do you have some sort of obvious defect (ie - third nipple or fingers growing out of the middle of your forehead) that your self-esteem is THAT LOW?
One silly little bimbo (she knows you have a serious gf and says she doesn't care) says I want to fuck you and you take this as some BIIIIIIIIIIIIIG ASS compliment?
I feel sorry for your girlfriend. Sounds to me that she is settling for far less than she deserves. Unless of course you two have an open relationship. Then I should think that you are BOTH free to fuck who you want, when you want. If that is the case then live and let live, stock up on the viagra, condoms for everyone!