To the best of my knowledge, my husband was not polyamorous. I've asked him a few times over the course of our marriage whether he was interested in finding other women, but I confess that I made it clear that the choice was only available after he divorced me. I was emphatic that I was monogamous and expected the same of any person I was involved with. I am not the best polyamorous candidate ever. Again, I am stunned that I'm standing where I am today.
To be fair, no one said, "I want someone else." To me, it was presented as, "Hey, I love you. We love you. We love you both. We want you in our lives permanently in whatever way possible." I don't know if my husband was seeking this--he assures me that he wasn't, and will only do what I'm willing to do--but I think it happened in the only way I would have been open to--friendship first, followed by growing feelings of love. No one pushed me for anything intimate. And, of course, no one even implied that I wasn't enough for them. Everyone involved went out of their way to be sure that I felt like they were desirous of me. It's quite heady to feel like three people are pursuing you! There have been a few times when I was quite jealous of my husband's relationship with our other female partner--not intimate, though, just the time they spent together--and felt that perhaps he would be happier with her. But I've always profited from their time together, because she has helped him understand my needs better and helped us improve our communication.
I think that I would have been like you, that if my husband had come to me and said, "Honey, I'm interested in seeing another woman" I would have come unglued. I can't think of any scenario that would have kept me from crying and screaming and throwing things at him. Even if I had eventually said yes, I would have resented him for not finding what he needed in me. It would have been the death of our relationship, even if we stayed married. This is definitely a case of right time, right place, right people.
I have to run out the door, but I'm enjoying this discussion.