It started with a simple threesome, a mutual friend from work. A guy we knew and trusted, and she found attractive.
Soon after the first threesome, I had to work away, travelling abroad for work for days at a time. She asked if she could still 'play’ with this guy and I told her yes, providing she didn't keep anything from me.
I've always believed that life is too short. I'm not arrogant enough to assume that she will never be attracted to or want to do things with other people. I want her to enjoy everything that life can offer, and I'm happy that she's free to do anything with anyone, as long as I know about it. A betrayal to me would not be her sleeping with someone else, it would be if she kept it from me.
I know there are people that will balk at this. And assume our relationship is heading nowhere, however we have been together almost 20 years, and are as committed to each other as ever. We trust each other implicitly, and can talk to the other about anything and everything.
As someone else has said in reply to this question — each to their own.