Cannot help the weird feelings, I would be really anxious in the boy's position (given this situation happens to me in reality). I don't know if I could honour my girlfriend after seeing her making out/having sex with her mother.
All through high school, my boyfriends always talked about my Mom and her awesome body. Two of them asked me to share her with me and as hot as it was doing things with them and her, this movie really made me wish that I had started even earlier with them and maybe even teach my Mom a few things about sex too.