I'm going to come at this from a different angle, ie. His. As someone who has been cheated on before, by ExH (many times), I can say that my tolerance for any nonsense at all, in a relationship, is Zero.
Your BF has been cheated on before. And in the worst way possible - you said:
he forgive his ex for getting off with his mate while he was out the room
That's an awful thing to have happened to him.
I think he has no tolerance for any whiff of flirting, and I am the same.
If my DH came home from a reunion, and told me that he had been chatting with a woman who had been openly flirting with him, complimenting his profile, and that they had joked about his cock size etc, I would seriously shut down. If it was 6 months in to the relationship, I would most likely withdraw, just like your BF has done.
To some people, this seems like a massive over reaction, but if you have been really hurt before, it's a snap reaction : the barriers come up. Not only did you have "banter" with this sleeze about your tits, you then relayed this story to your BF thinking it was funny. To someone who has been badly burned before, it's not funny. I would have done exactly the same. Taken it as a warning sign and ran.
On top of this, you told him that you are done. Another warning sign (no empathy for how he is feeling).