Here I go being a wet blanket, but perhaps his "concern" is merely a guilty conscious. I have known men (and women for that matter) that do what they want, but they don't want to feel like they're the bad guy.

They really believe that the person in the relationship doesn't understand/know them so they're free to find their happiness somewhere else, leaving the relationship partner in the dark. Once they find what they think they're looking for the cut old faithful loose. BUT, they're nice about it, because if they just did it the open and honest way, it would be painfully obvious what a heel they really are.

When you first posted about what you walked in on, I suspected how this would turn out. I've known four women and two men that have gotten a variation of the line he fed you. I can even give you the line he'll give her if she ever discovers what happened "oh she was in a bad place, I just felt so sorry for her. I didn't want to sleep with her, but she just wouldn't take no for an answer, what was I supposed to do?" (One of the women that was running this scheme ended up getting preggers by the husband on her farewell fling and ended up losing the husband she was leaving and the man she was leaving him for).

Until you confronted him about this new relationship, he had a foot on each side of the river. So now he's Ms. Newandexciting's problem, and he's already played her foul and they're still in the honeymoon phase of their relationship. What is he going to do when she gets boring?

Break ups really hurt, and they hurt for a long time, especially when someone has been blindsided the way you were. But in time you will realize the best thing he ever did for you was leave.