moonandstars,
Some people, like me, just know they are poly and have operated that way from early childhood. I always loved more than one person and regularly dated in twos and threes.
Other people, like my boyfriend, are flat-out mono. They never fall for more than one person at a time. In his case, there has only been one person EVER in his life. He's not even one who loves one person at a time, but one person, period. He's been in love with me since he was 17 years old.
Still others are like my dear husband, who were taught that monogamy is the RIGHT way to be during marriage and that any experience with more than one person needed to be had prior to saying "I do." He has poly "tendencies" and can be happy and enjoy sexual experiences in controlled-group situations, he can love more than one person to a degree, but he's conditioned for a monogamous relationship.
My boyfriend is not likely to change. Like Mono on here, he doesn't see the point. He would rather be alone than try to be with someone else.
I am not likely to change either. Despite trying, I simply DO love more than one person at a time, and when I try to block the emotions for one, what I accomplish is simply shutting down all of my emotions across the board.
My husband flip/flops. He MUST have security that he IS loved and that no one is going to leave him. He CAN be happy in a monogamous relationship, but he also can enjoy having more then one relationship. So, it's really a matter of ensuring that he's TREATED right, his needs are met and that he's happy, not what type of relationship it is. Does that make sense?
I think you are asking, "Is it this or that?" The answer is, it is both. Some people are wired one way or another, and some people are trained to be one way or the other.