I'm one of the people who opened up my marriage.
What happened for me was that I had been told so much that when you are really in love, you stop loving others, that I assumed it would happen to me too. Even though I had always had several crushes, or several boyfriends, I figured I would find "the one" and settle down and everything would be fine.
Add to that the fact that we needed to get married for him to be able to stay in the country, and we married much earlier than I otherwise would have wanted (although after three years, which many people consider normal).
When I kept falling in love for others I did my research, realise I wasn't evil or anything but just polyamorous, and told him about it. He was shocked and surprised, and hurt, being monogamous himself.
I absolutely understand the confusion and hurt the people must feel. Finally understand what's "wrong" with you is liberating. Being unable to explore that part of you is restricting and can make you depressed and miserable.
And people being what they are, each side is going to think of ways to change the other's mind. I've seen it in other occasions, such as children ("if he loves me, he'll realise he's happy with me and won't want children to ruin it!" "if she loves me, she'll want my children!"
I think it's important to realise that people might adapt, but rarely change to that extent. My husband reluctantly agreed, nothing came out of it for 1-2 years, then he met someone and brought it back up, then I met someone (Seamus, I'm with him now).
I had been with Seamus 6 months when it became obvious my marriage wasn't working, so I broke up. Two month later I learned that my husband identified as monogamous again, and said he had always been and was pretending for my sake.
In hindsight, I wish we had broken up the second I mentioned polyamory and he was against it. The rest of the relationship was a wreck, a mess, and we could have gone each our own ways still friendly if only we hadn't tried to hold on to this utopia of making it work anyways.
But sometimes it works, and people who don't know how to deal with these situations can use the help and advice of this board.