I thought he really loved me??

Mariana
Help!!!! I just found out my boy toy of 5 months is fucking my mother!!!!! I realized it when she was "helping him with school" and they started to talk without me. He finally confessed it's been going on 3 months! Should I stay with him? He says he is open to having sex with both of us. But I'm not sure I'm willing to share.....thoughts?