He's cheating on his girlfriend who's right next door
I think this is one of the naughtiest things I've ever done in my life, and that is that my sister-in-law and my sister slept in the same bed, and I couldn't control my urge to fuck her boyfriend because I love this guy and he drives me crazy. My hands started touching his crotch, and I noticed he was comfortable, then he got excited. In fact, I managed to sit on his dick and started moving gently. But he still didn't know I was his sister-in-law, but when he realized it, he didn't want to stop either. He just asked me to be more careful, but he penetrated me much harder.