In that situation, if I genuinely liked the girl I'd have given her another chance, maybe she wanted to seem keen (relationship wise) after you did the deed and would have toned it down over time.
TBH seems like he dipped his toe before he was really ready, got rail-roaded and is now back pedalling as fast as he can and feels like a complete douche bag for it. He should, he is.
But it isn't fatist. The thread would probably have gone another way if she was a fitty, but I'm not sure his reaction would have.