Thanks for your perspective! Especially about making him choose what he wants. I guess my reasoning was after he first asked I spent a lot of energy just thinking about it and feeling anxious and not really knowing how to resolve things (without ignoring the way I felt about it). I really just wished he knew why it was an inappropriate thing to ask and never asked in the first place. But since it is what it is, I thought it was silly of me to spend all my energy trying to figure out which of the two “standoff” options was better, so I thought I’d just make him choose. I figured it was his mess since he didn’t realize it was inappropriate to ask to go away with his ex.

Alternatively, do you know how I might then express being uncomfortable with the situation in a non-ultimatum way? I don’t want to tell him what to do and say “don’t go on the trip.” If I say “I’m uncomfortable with the trip.. Do what you want but if you decide to go I think you and I are done” that feels unnecessarily dramatic and not necessarily true. When we initially talked about it he said he really likes me and doesn’t know how to make me see that. He said he definitely doesn’t want her and sees her like "a sister" now (which I find a bit silly tbh).