Most Helpful Guy
Sounds like she feels like you're taking him from her. Sad thing is, women like that give good women a bad name. As to why he would cheat with her. She had his kid. That makes him linked to her weather he likes it or not, plus there might still be feelings there. Be prepared to deal with Parental Alienation. Women like that will use children as bargaining chips to manipulate the father. Hopefully he has a good lawyer.
A very good friend of mine was in a similar situation minus the sex. When she found out that he got married again she literally stalked him and threatened to kill his new wife. Additionally she went as far as telling him that he would never see his kid until he divorced his new wife. thankfully he took my advice and recorded all of their phone conversations and saved all of her texts and emails. When it came time to go to court to negotiate visitation he walked away with full custody and a restraining order against her. I think the girl is in prison now.. can't remember why though.
Most Helpful Girl
She gets to have her cake and eat it too? She clearly enjoyed having sex with your now husband, and she got to have it while still having her husband. She's upset that her perfect, having her cake and eating it, scenario has now ended (due to you). Perhaps she will let him go at some point, when she finds someone else convenient to replace the role he had in her life, perhaps she will never, it's difficult to say.
As for the last question, that's one you should ask him. The most we can do is speculate. Perhaps he assumed her husband knew and they had an 'understanding'. Perhaps it just didn't occur to him because socially/culturally cheating is acceptable. Perhaps he just didn't want to look a gift horse in the face, no strings attached reliable source of sex.