For us, I (husband) had briefly been in a poly triad before meeting my wife. She knew about that, and thought it really wrong that my girlfriend was cheating on her husband. I patiently explained that she wasn't cheating, and I was friends with her husband, and everything was on the up and up. She still didn't like it, and was adamantly opposed to the idea. I never suggested it for us, I just shared it as part of what couples do; sharing their past.
Over the early years of our marriage, the subject of the poly triad I had been in came up every once in a blue moon. My now wife had always voiced objection to it. I would have bet every penny I ever earned that we would never have gotten into swinging or polyamory. It wasn't even a thought. I was quite content with things how they were, and being monogamous with my wife for the rest of my life.
Then, six years in, she surprised the hell out of me and mentioned in bed one evening something like "You know, I could see the advantage in having two men; both could massage me at the same time" (she absolutely loves massage; can't get enough of it). That started the ball rolling, and many months later after considerable discussion, lots of reading together, thoughts, scenarios considered, etc. we found ourselves at a swing club for the first time ever. The first few experiences weren't all that great. Not bad, just not great. Then, she had a lover who just made her toes curl, and she completely bought into non-monogamy then. That was 12 years ago now, and we're still here.