I had this same dilemma around this same time last year. One of my undergrad joints came through to my spot to kick it. It started innocently enough. I had a night class and she was in town for work, so she was just dropping by for a few mins. We started talking about work, I told her about grad school, etc. then one thing lead to another and she suggested we go to the liquor store for a bottle of Hennessy... You know... For old times sake. I knew where it would lead, and figured I could take a shot or two and catch a quick one before I went to class. She gets the bottle, we pour up, and two shots turns into us killing the bottle (I always admired her drinking skills, she wasn't a lightweight). The talking turns to touching, and we get busy. Zero dambs given about the class. She was always this cool girl with me. The first girl I ever smashed in college actually. We had that type of friendship where she knew not to expect anything serious from me. She knew I was out there getting it in with other girls and didn't mind for the most part. Tried to get into a relationship with me one good time but I curved the idea since I knew it'd never work. So after we get our rocks off, we lay around in my bed and she goes "that shouldn't have happened". I hit her with "well it's never supposed to happen, but we'll be alright". She shakes her head and say "no... This time it really wasn't". Then she goes into the spiel saying she's met someone, she found the ring, and she confirmed that dude was planning a wintertime engagement. I felt bad as hell because I swore off sleeping with girls that were in relationships. In my defense I didn't know. Turns out she came over to my place to talk to me about whether I thought it was a good idea, whether I'd be mad, would I be at the ceremony, etc.