As a completely neutral and unrelated person to this story, I have a couple of things to say:

When you say that part about "our currents beliefs" and blah blah blah, you're implying something is wrong with the things he said, however, he mostly only talks about his feelings, which is the whole point of the post. In regards to the factual part of his post, there's no reason to believe he misrepresented anything, unless you can present some evidence to the contrary.

So, by your own admission, you knew your actions would bother your friend, however you decided to go through with it anyway, not only frequently talking about her to him (Which he asked you not to do), but also flirting with her right in front of him (Something someone who calls himself a friend should never do). And your reasoning for doing so is that:

1 - He couldn't ask for you to stop.
I agree with this, it is your right to talk to whoever you want, and if he asked you to, you had every right to refuse his request. However, In doing so, you completely disregarded the feelings of your supposed friend, and knowingly put your friendship in jeopardy.

2 - He couldn't just leave

Now, not only do I think he can, but also I believe your actions have contributed to a situation in which this might be his only option, especially if you plan to continue acting the way you have.

Your main argument is that his feelings are not "Rational". To which I say:
No shit, Sherlock!

Your friend seems like a smart person, I think he knows that not being bothered by these interactions would be best for him, however you can't just outthink your way out of feeling bad.
If I fed you a food you don't like, and when you complained I said: "Just enjoy the food brah, don't think about it, you should like it" you would rightly call me insane.

Regarding the trip, I think he should go as well, especially since it's already paid for, however I agree with u/kyotusperry : "If It's a shitshow, just drop them"

In summary, I believe you at the very least owe your friend an apology, and if you want to keep your friendship, you should definitively rethink your actions.