If you are to get this transition, there can be no ambiguity. Instead of being clear and assertive about their intentions, they keep trying to send weak signals that are not being understood by the other person.This ultimately leads them to more pain and frustration.
When your friend turned down your request to fly back from NY together, he clearly signaled he wanted to make a clean break.
I’m sure it was difficult for you to accept that he no longer valued even the remaining vestiges of your friendship.
My guess is that he didn’t want to embarrass you, himself, or others at the social event---either by ignoring you or going into the details of your prior relationship.
People often use the term “friend” quite loosely; it includes people who are more distant acquaintances.
When a relationship has a history of being platonic it is unlikely to change barring a substantial change in personality or behavior by one or both of the people involved.
Since this is necessarily an "unnatural" thing, no, I don't think it tends to happen naturally unless there was sexual chemistry from the start.
Listen, you should take a good look at how you have approached this entire relationship. What impression did you give this person about your intentions?
The biggest mistake that people make in this situation is procrastination and allowing doubt to impede their judgment. You can’t get yourself to a relationship through guesswork. Your friend isn’t going to understand the weak signs that you are sending them.
He introduced me to his new boyfriend as his friend.
I wanted to tell him face to face that we were not friends at all any more but decided to save him this embarrassment in front of his new boyfriend.
Generally speaking, you are better off letting a woman know you're interested in her as soon as you are sure you are. Not only will it not affect you as much - it will happen a lot less often.