For simplicity, we generally say person can be homosexual, heterosexual, or bisexual.
I believe a person can be mono, poly, or poly-mono-flexible.
I've known people who simply cannot do monogamous relationships. If they're forced to try, they inevitably end up cheating, despite their best intentions. They've got no problems with communication and honesty when they're allowed to have other relationships, so it's not "once a cheater, always a cheater" it's just that they can't do monogamy, at all.
I've known people who simply cannot do polyamorous relationships. Part of what they like in a relationship is being the centre of someone's attention, and having that person as the centre of their attention. They can't share and they don't want to be shared, at all.
And through this forum, I've known people who can go either way depending on the preference of the person/s they're currently in relationships with.
I was definitely born poly, I have never understood monogamy or its appeal. But I don't have a "need" for any type of relationship. So to the same extent that I've never felt a "need" for a partner when I was single, I've also never felt the "need" for an additional partner when I was already with someone. But the mere thought that I'm tied to this one person for ever and always til death do us part sounds like nothing short of a prison sentence. In other words, if I'm in a relationship and I meet someone amazing that I would like to date, I would feel completely trapped if I wasn't allowed to explore that.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker