Girlfriend wanted MMF threesome?

Soooo long storey short, me and my girl were at a party drinking. She got pretty drunk

Near the end of the night, my girl jokingly said, "hey we should have a threesome". I knew it was a joke so I played along with it saying, "yeah sure."

We go into a room and we start making out. She started taking this "joke" to far by taking off my pants and sucking me off. While my friend was present in the room.

Needless to say I shoulda stopped it sooner.

She starts asking me to take off her clothes but I don't. I then reply with "do you actually want to do this?" Which she replies with "I wouldn't mind".

This is when she asks my friend to kiss her neck, so he does. I'm really turned off by this point cause I thought I knew this girl pretty well. I whispered to her like 3-4 times to stop and eventually say it loud enough that he hears it and he stops.

He leaves the room and we start to ****.

I'm just wondering how others would react, if they thought they knew their girl pretty well and she pulled this? What goes through your head? For me, I don't want some other dude railing my girlfriend.

She feels disgusted with the way she acted in the morning and I told her I woulda done the same thing if it was MFF. So I can't blame her.

I'm still angry at her and disgusted too. What do you think about all this, and what would you do/have done in that situation.

I know if the threesome did take its course, that I could no longer be with her. But all this puts doubt in my mind now.