My fiancé's best friend

We all were at my house drinking, my girlfriend went to bed - her friend and I stayed up and she came onto me kissed me on the neck - grabbed my dick kissed my lips , I grabbed her titties and hips , she pulled my dick out and started sucking me then she got naked , made me finger her from the back while she kept going , then jumped on it and made me cum inside her and kissed me after

My girlfriend has no idea , and we're going to keep fucking , am I the asshole ? She has a great pussy