My wife did about a year ago. We've been married about 7 years at that point. No issues with our sex life, she just found another guy she was emotionally attracted to and I gave her permission to pursue the relationship further.
I've considered pursuing a polyamorous myself, but I realistically don't have the time in my life at the moment to fit in a second partner. So I occasionally see escorts provided I tell the missus I'm seeing one.
So far our sex life hasn't diminished. A couple of months after she had a second partner, my wife and I saw an escort together and had a threesome. My wife also seems to orgasm mostly through sex alone. So I'm not complaining that she has another bloke in her life.