OK, I feel a bit mean cause of course I'd go to Doctor's appointments with friends and whatnot, but still, I wouldn't cancel plans with a partner on the ONLY day of the week they were free when it was known to everybody that it was our date night unless it was really important.
I am a bit concerned that (maybe more than a bit) about what you say about some of his other partners being unhappy. Have you heard this from them? Are you communicating with them? If you're hearing it from him, do his other partners say it's fine that he's discussing their private feelings with you? It also is possible he is handling his NRE with you so badly that he's not treating them as well as he has in the past, and that's why they aren't happy. I personally feel him talking to you about wanting to break up with the one woman is a red flag. How would you feel if you found out he was talking to his other partners about you like that some day?
Do you KNOW any of his other partners? I think that it is great you and he are happy together and all, but it seems that if you are discussing marriage when you haven't met his children or the other important people in his life, just may mean you two are living in a giddy cloud of NRE and not being realistic. (Maybe you have met them, but from your limited time you have free it seems like probably not - I've heard often that meeting metamours helps bad feelings from forming, when you've met this other person and know that they are nice and kind and not a threat).
I think since the tickets have been purchased, maybe you need to go ahead and go through with it. You DID ok the idea after all. Maybe seeing the realism of his life, and how he handles and juggles his partners, time and actions would give you a better idea what to expect if you end up building a life together.
Happiness will never come to those who fail to appreciate what they already have.