Thank you for clarifying.
I knew that Brian was having PIV sex with his girlfriend because he came into the room where I was with my boyfriend and actually got a condom out of a drawer, right in front of us, without saying a word about it.
That seems like missed opportunity for you to say "Um... I thought the promise was no PIV sex? What's going on?"
Brian then proceeded into the next room, where his girlfriend was. He left the door to the room they were in wide open and my boyfriend and I could hear them going at it. If it had not been dark in there, we would have seen everything too.
I get your upset. You were stunned. But again -- missed opportunities to close the door, stop and say something, been more clear before going over, stayed home and NOT gone over, spoken to the GF (your meta), etc. Many options.
Basically it's a situation you and Brian both had a hand in creating from botched communication.
At this time what is the greatest need? Is it that you need him to see/empathize with you that you are hurt?
If so...could ask "Could you please see that I am hurting? Let's not quibble about HOW we got here. This is just where we are at.
Could you see and acknowledge that I hurt, and that I want to talk about changes moving forward from here?"
Could choose to improve the communication and move it forward.
Or choose to keep it in the hamster wheel stuck.
Hang in there.