I think a lot of people feel their personality, soul, or what have you leads them naturally to a poly lifestyle. Even if you're not in a poly relationship, you're still open to the idea, and accepting that part of yourself that says you can fall in love with more than one person at a time.
I was poly in college (though I didn't know the term then, or that there was anyone else on the planet like me). At some point I figured I would be alone the rest of my life if I couldn't commit to one person, so I forced myself to that. It didn't stick. I got married, and I still am very happily married, but we had a big falling out regarding polyamory, once I found out more people were out there like me than I thought. Now, my partner accepts me, and I'm open to falling in love with others. I don't HAVE to be in a poly relationship to BE poly. I just have to be willing to fall in love, and do something about it aside from turn away because I'm already married.
So, I think the same applies for single people. It's better they be open about who they are with their partners, so that they don't end up in a mono relationship that's comfortable for years until they suddenly fall for 2 people at once.