When my girlfriend first moved in with us, we were planning to save up money and move out together. It took longer than we hoped. She was having a hard time finding a job, and I had to work nights. After a few weeks she got pissed about not being able to wear her normal tshirt and panties around the house and decided to just decided to wear it after he went to bed. When she went to refill her water bottle one night she came running in all blushy and said he was staring at her while she was bent over using the britta. About a month in he started making flirty comments around her about her and at her. At first she’d eye roll or tell him to knock it off. One of them was about her needing to “cover her cheeks” I felt bad for her and he said he’d “ease up” on her. In the meantime our sex life got kind of strained. I would come home very late and start work mid afternoon. I started noticing that she wasn’t reacting as shocked by his comments which only seemed to ramp up. Over time he started to get more and more brazen in front of me. I noticed lingering touches from him. And noticed her seeming to lean in. One night she made a comment when we were going to watch a movie and it looked already watched I said something to the effect of l”ooks like you already watched that” she responded “yeah your dad had it on but I was on my phone most of it” I was surprised throughout the movie that she seemed to be seeing it all for the first time. Looking back that was a sign. I was still thinking she was just being flirty or dealing with my dad since his teasing of any kind is just relentless. One night I was working and she said she forgot her towel in the room and that he was watching TV in the living room shed have to walk through I said that I’d be home soon and I’d bring it to her. My work got busy and it looked like I was stuck there a bit longer. She said she’d throw her shirt on and dry when she got there. By the time I checked it had been a while. When I got home she texted me she was putting lotion on and reading so I sat down by my dad to watch tv while she did her thing. When I got up I noticed that my grey sweat shirt was very wet and headed up to hang it up and headed up she was in the bathroom I opened our laundry basket her sleeping panties caught my eye they looked very wet I took them so that that they wouldn’t make every thing else wet and noticed they were sticky. I knew instantly it was cum. And putting things together I confronted her. She told me it had been going on with him for a while and that she didn’t plan to stop. I slept on the couch that night too angry to talk to my dad or her. In the middle of the night I woke to find that she was serious and they were fucking in his room I found myself horny and conflicted I finally allowed myself to cum to it and they were still going that made me horny again. That very fact embarrassed me. I allowed it to continue feeling too embarrassed to say anything.. I eventually yelled at him when the three of us were drinking one night. He basically just started grabbing her ass and asked what I was going to do about it…