I have had two situations like this in my life. the first one was before I was married and had briefly dated a cheerleader from a local college. I was unsure of where the relationship was going and my best friend at the time ended up ****ing her one night at a party. He was terrified to tell me but when he di he was surprised at how I thanked him and was grateful that he did. What he actually did beyond ****ing my girl was show me that she was not the right girl for me and saved me the wasted time dating her any longer. he did me a favor.
The second time was when I was staying over at a different friends home after a night of drinking. my friend was pretty much knocked out from the drinking and we had all gone to bed. I was sleeping alone on the couch when about an hour after going to bed his wife ended up waking me up and was standing completely naked next to the couch asking me to have sex with her. I respectfully told her how incredible she looked but respectfully declined and she went back up to bed. The next day she called me and was crying and afraid that I was going to tell my friend. I assured her that since nothing happened I would keep it between us and have never told my friend. Turns out though that my friend was actually sleeping around and the wife was feeling neglected. They divorced a year after that night and both have remarried. I have never told anybody about that night and although I am no longer friends with my friend I am still good friends with the wife and there has not been any sexual tension or flirting ever since and that was over ten years ago. Sometimes people make mistakes. In your case however, you are just taking advantage of your friend and his GF. Sleeping with your friends GF one time was one time too many and because you went back for more tells me that neither you or the GF gives a shit about your friend.
The best advice is to not seek advice from the internet.