I have been in your girlfriend's shoes and I would love to tell you about my feelings about it. My boyfriend and I started dating at 19, about a year later we took a break due to issues that were coming up from both sides. It was really tough for me but i knew the best thing would be to try and move on.
After a few months of moping, I made an okcupid account and hit it off with this guy who was 25. I knew he was player. We used to go on dates and he'd tell me about other girls he was dating. Not a real winner at all. But I thought he was sweet in his own right and he did seem to care for me. A few months later we had sex. I was a virgin at that time.
Meanwhile my exbf had found out I was seeing this guy and had changed his mind about me and set on the track to winning me back -which was super easy for him. He was my high school crush, my best friend, my first boyfriend, my first kiss. So we got back together and I stopped seeing the other guy. We are still friendly though and check in on each other through texts.
I do have real feelings for the "player" guy. Not romantic feelings, but he helped me through a really bad time in my life and helped me rebuild my self esteem, made me feel sexy and desired again. I am NOT attracted to him any longer. I consider him a friend who is no longer a party of my every day life but he hasn't stopped existing for me.
I don't think about him all the time. Probably a passing thought like once a week or if he texts me. The feelings I have for my boyfriend are real and true and strong. I would never give him up for anything.
I don't have that much advice except to listen to your girl. She probably has similar feelings as I do. I know it sucks and personally have basically stopped talking about it to bf. But I have always been 100% honest with him and he believes me. In the end she chose you. There is a lot more I could say here but all I will say is to consider yourself lucky to be with her, appreciate her and don't get hung up on this thing which could easily become something that will create distance between you two. There's no reason for that to happen. She loves YOU. That guy has come and gone but you are the one who is her constant and her support. Never think that a little physical fling can begin to compare to what you have with her.