Massage Led to Penetration of My Girlfriends Stepmom

Wow, my girlfriends stepmom asked me to rub her shoulders, but I started to get an erection. So I rubbed her shoulders, but then I moved down to her back, and then down to her lower back, and then to her ass, she started moving and grooving and grinding her big booty into my dick, I think I accidentally made her horny and we started to fuck. Please don’t tell my girlfriend. Am I bad? Because I want to still keep fucking my girlfriends Stepmom.