You are a slut.
No she's not.Good for her,it's not hurting anyone
That is disturbing. I think your bf may harbor some resentment towards you or women in general. You may have cum, but you were put in an embarrassing situation and were taken advantage of. If you said no way, would it have ended there or would his friend still fuck you while you were tied up with bf looking on? It is just strange that he wouldn't tell you what his fantasy was, just had a friend walk in without saying a word about it. It is kind of demented. Good luck whatever you do, it sounds like you have a sadist on your hands.
Getting put in an embarrassing situation and taken advantage of, is what gets my juices flowing. Not knowing what is going to happed to you sexualy, is what makes it kinky. You have to trust the person that's putting you in that position. I would never dream of doing something like that with strangers and get beat up in the prosses.