There are plenty of repentant OP's on this board, and though I like their perspective, talking to someone who is in the thick of it and feels no remorse will really help up in understanding the OP and what they might be giving our spouses.
If you consider the OP your enemy, be grateful that you have a real live one here who can help you understand them. Know your enemy.
And if we are honest with ourselves, they are not evil, not anymore than we are -- if we were pushed hard enough.
My wife is an EA with my ex-best friend. They both claim they are "just friends". There are moments when I think of straying and running off with someone else just to feel loved and cared for. And guess what, I don't think about my kids when those thoughts start to rear their ugly head. It's so easy to be narcissistic in times like this.
I think Lissete's post, however, should be framed. It expresses how we all feel when we think of how our kids will be hurt by all this.
Perhaps our original poster needs to consider that this man is in a good relationship with his girlfriend and loves her. He's not escaping a bad situation. For the sake of the little girl she should walk away.
Shirley Glass said affairs happens to good people in good marriages. It's just a matter of letting our boundaries slip (which is a choice) and then, little by little, you connect with someone else and have the affair. It sounds like this man let his boundaries slip and now, under the chemical influence of a new romance, can't let the OP go.