When I was 18 my gf and I were staying back at my dad’s when we were at uni. We had been out with some old friends getting drunk and had headed home late in the morning. My dad is a late slepper and was still up when we got in. We both said goodnight and headed to bed but I woke up a while later and realised my gf wasn’t with me in bed. I didn’t think anything as she probably just went to the loo, but I got curious when she didn’t come back for half an hour. I slowly crept downstairs and could still see the living room light on and some muffled noises. I kept as quiet as I could and poppwwmy head round just in time to see my gf finish blowing my dad and climb on top of. Him on the sofa. Now I’d seen my dad around naked before but never erect and it was clear he was over twice as big as me!! My gf couldn’t help but let out a little scream as she took the biggest cock she’s ever had. I don’t know if it was because I was still drunk but I immediately got hard but didn’t dare touch myself as for some reason I didn’t want to disturb them. My gf was really fucking him hard and eventually I could sense he was going to cum inside her as she orgasms on his cock.