Can Ex Lovers Just Be Friends?

Three years ago my wife met a man that she was very attracted to and wanted to be friends with. Initially, she said that they had a spark but it wouldn't go past friendship. I questioned this and we argues but I eventually said ok. After a few weeks she said that she wanted to have sex with him. Bigger argument. We had discussed sexual fantasies and even possibly having some open relationships together while in bed but never decided to and even if we did would have limitations such as only with people didn't know, one time things together and not with single people. He was single etc. But I also gave in. She did she then lost interest in me and only wanted to be with him. We were like separated but living together for about 2 months when it came to a head and she stopped seeing him as a boyfriend so to peak. She however wanted to remain friends. She was emotionally attached to him. Although she has had less and less contact with him she still does and when she does it is always an issue even if it is just a text or a phone call. I have a hard time not being angry insecure jealous and scared. We have not opened our marriage or done other things although I did some online ***/chatting while she was with the man she met. Since she was open and honest with me about her interaction with the man (I was not open and honest when I was ***/online chatting although she knows now and it is an issue for her that I wasn't open with her about it at the time), should I just let it go and let them be friends (not sure I could really control it.