My answer is a bit different...or maybe a lot different.
When I met Hubby almost 8 years ago, I asked him what he would think of having an open relationship. He said he wouldn't be willing to do that. We became officially exclusive about a month after we first met (we became a couple that first night; my life works that way sometimes), and were monogamous from then until about 3 years ago.
I'm one of those who believes being polyamorous can be a choice *or* an orientation; for me, it's an orientation. It's what has always made sense to me, since I was old enough to think about relationships at all. But I didn't have a name for it, and I didn't act on it because that isn't how things are "supposed to be." I was miserable living monogamously, but I did it because most people are taught that's how they're supposed to live.
Three years ago, I confronted Hubby about some serious discord in our sex life. He's open-minded, but for himself is extremely conservative sexually, which meant he was shooting down anything I suggested that didn't involve being in our bed going from foreplay to intercourse. For reasons I won't go into, aside from being quite boring, his refusals to try things and the judgmental comments he made (which he's since admitted were him trying to cover up his discomfort) were very emotionally damaging to me.
When I confronted him, he thought about it overnight and the next day told me he was okay with me seeking other sexual partners if it would make things better for me. So I did. Six months later, I realized I'd fallen in love with one of those other partners, and per the agreement Hubby and I had made, I told Hubby "I'm in love with Guy, and I'm going to cut off all communication with him, but i wanted to be honest with you first."
His response was, "I don't want you to do that. You loving Guy isn't going to take away from how much you love me. You're polyamorous, and I'm okay with that." (He was the one who put the word to it.)
So tl/dr version: My monogamous husband was the one who identified how I felt as polyamory, and he accepted it immediately--faster and more easily than *I* did.