You were informed of this in your other thread, but I think it bears repeating since you say you think it is a "recurring theme" in polyamory : group sex is not automatically expected nor a requirement to be polyamorous. Repeat that over and over to yourself until it sinks in, because you don't seem to believe any of us that have said that to you. Polyamory simply means the ability to love more than one person. That doesn't mean all people involved are expected to be fucking in the same room together, and it doesn't even have to mean sex at all!
You are like the girl in high school who saw only the good things in the bad boy from the wrong side of the tracks who was stealing and setting things on fire. Everyone else could see he was trouble and not good for you, but that naive girl who just wanted affection and was flattered that he wanted her, kept focusing on how sensitive he was underneath it all, and how no one ever really saw his "nice side."
Over and over again, this guy has dismissed and disrespected your wishes. When will you stand up for what you want/need and realize that if you do so and it closes the door on this relationship, you make room for something better and much more respectful to come into your life?
The fact that you know you don't want it, and that you are afraid he will push you into it, is a huge red flag. Trust your intuition - he will probably push you. He is like a baby who only wants what he wants. I still say you should cancel the trip in order to send the message that you are an individual with your own needs, wants, and boundaries that must be respected. Then see what kind of tantrum he throws.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein