The "rule" I have had with all my girlfriends and wife's is that I do not own them, they are grown ass women and if they want to fuck other men they are free to do so after they have notified me about it, at least as soon that they have done it so I know it.
I once had the same "problem" with my first girlfriends when I let her have sex with my best friend, that where a virgin at that time. It was only suppose to happen once, and when he fucked her with a condom and came in her I thought that was it, but later that night he fucked her again and at first I was a little annoyed by that, but thought better of it. The cat was out of the bag, or rather my girlfriends pussy was out of her underwear so he ended up fucking her as much as he liked that weekend, and in all the three years I was steady with her. So much for only once...
After that I was very clear in what I expected and where the borders should be...