I'm going to link an article for those who think sex is some evil act to do with a friend. (only did it once). The article mentions that sometimes friends do have sex and end up going back to be friends. We realized we weren't compatible in a relationship as my sex drive is much, much higher than hers (weekly minimum for me, for her it's more than monthly) But she's a nice person, and our personalities are very much alike, so we are still friends.
http://www.psychologytoday.com/articles/200109/can-men-and-women-be-friends
I should note he doesn't know I had sex with her. She never told him as she was single at the time, and it's not his business.
I have come to accept I cannot see her. I can understand why he's jealous. She has more fun with me, than with him. She only went back to him for the kids, she doesn't actually love him. But he had full custody over the kids, and she can't risk losing visitation. He basically manipulated her into going back to him. He said he changed his controlling ways, but soon as he moved into her house he restricts her work and friend activities.