There's a slang term used by polyamorous people: cowboy. Or cowgirl, as the case may be. It refers to a monogamous person who meets someone who openly identifies as polyamorous, becomes romantically involved with them, and attempts to "cut them out of the herd," meaning: persuade them to sever existing relationships and embrace monogamy. This term is not a compliment.
I understand why people—including those dating the person being courted by the monogamist—view such tactics askance. The number of potential partners is small enough for polyamorous people without the mono crowd rustling them. It's also no mystery why they try. Viewed through a monogamist's gaze, dropping your lasso on a wandering heart is the stuff of songs, literature, and drama. But it begs the question: Why the hell would a poly person get romantically involved with someone who is clearly monogamous in the first place?
The honest answer is something like: hormones, misguided optimism and willful self-delusion, more hormones, and a little emotional masochism—or maybe more than a little. Most romances—monogamous or otherwise—don't turn into long-term commitments, and there are many reasons why. But I can promise you, if you're poly and you're involved with someone who's not, once the hot sex cools off and reality sets in, every single problem that occurs in the relationship will somehow devolve to: You're fucking other people. If you would just be monogamous, then your partner would stop drinking too much, bring you flowers, tell his family to be nicer to you, and always pick up his dirty clothes off the floor. And when this romantic idyll ends—as it probably will—it'll be just another example of how polyamory ruins relationships.
Before mono people get all outraged, let me say that, yes, some poly people break hearts because they won't forsake all others. I was one of them, before I realized this was one of my nonnegotiable features. All jokes about dominatrices aside, I actually don't like hurting people—or being hurt by them. My next strategy was to tell people I was poly before there was so much as a kiss. If matters proceeded... well, that meant they accepted the situation, right?
Wrong. Some monogamous people sincerely, but mistakenly, thought they'd be fine with me. But I have met a lot of cowboys and cowgirls. I vividly remember an outraged lover yelling, "I know you said that, but I thought I could turn you!" Another man said, "I viewed you as a challenge." It's painful to be honest about such an emotionally loaded subject, only to be met with mendacity in return.
It doesn't really matter whether you think monogamy or polyamory is more natural or evolved. Dismissing people's stated definitions of their sexuality as something you can make them change is not love. It's just disrespectful, and it usually ends badly anyway. So put the lasso away, cowpokes, and if someone says, "Don't fence me in," believe them.