This is going back quite some time now, but I used to fuck my girlfriend at the time, sister. We were dating for about 3 years or so and it started just over 2 years and in.
I was 20 and she was 23 when it started. It began by when she broke up with her boyfriend and moved back home. I often stayed at my girlfriends house (it was much closer to both work and uni for me) and our work schedules tied together that we were regularly both at home alone together.
As you can imagine we grew quite close as friends (just friends at this stage) A few months down the track it was a Friday night and we both just so happened to have the night off, and the house was empty too. We bought a couple of bottles of red wine, and as you guess this was the first night we fucked. What started this escapade was when we were actually talking about my relationship, and how I was some what struggling with my girlfriends serve bipolar (she often never took her meds) It was after that we began to cuddle watching a movie and we were both drunk, I was extremely attracted to her and she was of me, and it happened, right there on the couch and living room floor. In the morning we both admitted neither of us regretted it and thought it was just a one off thing, so we kept quiet and tried to put it behind us.