My first wife agreed to it right away, and said she would, but it took a couple years before the situation presented itself. My second wife took to it right away also, while we were still dating, she did it. From there, we played with it as fantasy, with a couple of "almost happeneds" until she finally made it happen for real one night.
I've been married to my wife now for 18 years, longer than the previous two combined. She was skeptical at first, then agreed to do it but changed her mind (I found out later that was after discussing it with her mom, who told her she'd do it, but that she'd keep it a secret from me). Over the years, she would say that it was fine for me to think about, but that it would "never happen." After about 5 years, she one day, out of the blue, asked me if I would really be OK with it if it happened. A guy at her school was flirting with her. Nothing came of that, but she told me that she'd decided that if she had an opportunity and she felt like doing it that she would. It was about another year after that that it happened. Which is what she said to me when she called to tell me. "It happened."
A few years later, she began to read about it and watch YouTube videos, trying to understand. Some psychologist there explained that it often starts as the husband's idea, and that the wife has to be convicted. BUT that once she is convinced, she will forever want to do it. I'm not sure how accurate that is, but my wife assured me that it is absolutely true for her. She said it is freedom and to give up a freedom once you have it, no way.
However, my ex's, although they WOULD do it again, and enjoyed the fantasy, were reluctant. They didn't want to fuck just anybody, and the right situation doesn't often present itself. I find that to be true with my wife now as well. Although she's been ready a couple of times over the past year, and the guys end up flaking. They'll play with her tits and let her fondle their dicks, but then they stop short of fucking. Frustrating for me, although she enjoys the pursuit more than the actual sex, so it works for her!