My girlfriend shares me with her best friend.

The other day I went to my girlfriend’s house because I felt like fucking her. As soon as I got there, from the window of her room, I saw her together with her best friend undressing. I stood there for a while and watched them licking her pussy and masturbating. So, excited, I went in and my girlfriend instead of being surprised, asked me if, in addition to her, I wanted to fuck her friend as well. How do you think it turned out?