Not my gf but my friends gf, now his wife. We were all drinking and ended up falling asleep on his bed with her in the middle. I woke up to his loud ass snoring. I guess she did too because after a while she started grinding her ass into my cock. I let her do it for a while, then just said fuck it and pulled my dick out. She pulled her shorts down and guided it into her pussy. I fucked her all night and came inside her multiple times while my buddy slept right next to us. The next day we both acted like it never happened.
Years later I went to their wedding and couldn’t stop thinking about that night.