boyfriend has a mom daughter kink

I accidentally crossed a line in the bedroom my boyfriend and I were having sex and my mom was at the door asking questions and I was answering here and she walked away. I turned to him and said I bet she could hear us fucking and that turned him on so much he wouldn't stop talking about doing stuff with both of us together. So the next day he still wouldn't stop but it's getting worse he asked me if I would let him have sex with my mom and now he is being super weird. I told him it made me feel weird and he said it was my fault because now he has all these bad thoughts about my mom. I am super fucking pissed that he just keeps asking about us together lmao. I know it's a fantasy and he said he would never act on it but still what the fuck.