I think people can program themselves to be polygamous in a couple of different ways. I'm not saying people do this intentionally, or that people can't be that way naturally (ie. not trying to start a flame war, to each their own!), here are some of my thoughts.
1. Broken hearts
They totally believe in monogamy when they're young. They dream of that perfect man sweeping them off their feet. It's going to be so easy when the right one comes along! It'll be happily ever after!
Then reality hits, and they find relationships take a shit load of work. They put their entire being on the line of their first romance, and get tragically crushed when it doesn't work out. After investing years of their life they swear off committed relationships because "they never work".
Really its just a broken heart and waking up to the reality of the world.
We can watch thousands of people fuck, look at thousands of cocks and asses in a matter of hours now a days. In the same way that children's television programs have become more and more frenetic, we as adults have shortened our sexual attention spans right down to nothing...
If you keep looking at porn, keep cruising men online, when you're in a relationship, it's easy to start thinking that the grass is always greener with someone else. So when any relationship hits rocky times, you just turn to the attention of other men online to make you feel better about yourself. Suddenly you actually want to BE with someone else. *Ding!* Next please!
3. Attention & Validation
Most gay men have rough childhoods. We hide who we really are for nearly the entire developmental period of our life. When we do finally come out, and we find that people actually accept us, we're floored! We've hated ourselves for so long, it's great to feel someone else likes us. We like it so much in fact, that we get hooked on the feeling.
Seeing as most of our parents never accepted us, especially our dads, if they were even around to care, this new found sense of having other men validate for who we are is intoxicating... but in truth, it's never quite enough. It never really fills that hole that was made when we were kids and really needed someone to accept us for who we are, for the parts of ourselves we kept hidden because we knew they were unlovable...
You see I think the biggest issue with men not wanting monogamy isn't the fact that they can't love one other person, it's that they believe others can't truly love them. They need to have other men tell them they are worthy, they are like-able, love-able. What better way to get that then from the intimate act of fucking? If a guy gives you that he must surely like you, right?
Once is never enough, cause that feeling goes away when it's all over, then it's onto the next dude. That rush of excitement when he shows interest, the sexual tension and build up, waiting for that moment when you can finally have him...
Anyway, these are just my opinions that I've garnered from some reading, a decade of experience and some thought. I suggest you check out the book "The Velvet Rage" which helped me put a lot of things together for myself.
Ultimately, if people really feel they're polygamous and they're set on that, I think you should move on and look for a monogamous partner who's long term goals match yours. It's really everyone's choice and freedom to go after what they want. Trying to force a bf to be something he feels he isn't will just hurt both of you.