My slutty gf getting shagged by my stepdad on a couch

Not sure if my gf and stepdad have been fucking all along or he coincidentally seduced her when she was horny and weak so she fell for it. He kisses her passionately then takes her on the couch and fucks her really good until she climaxes. I'm afraid she won't like my fucking after this kind of action by my evil stepdad.