I know that when I was in my first years of sexual activity as a teenager, I wanted to break all the rules. I remember feeling very sad once, there was a boy I adored, he broke up with me, over the course of a year or more after that as he dated others he still snuck over to my place to have sex occasionally...and one day he met "the one" (he did end up marrying her!) and I liked her too, and I wished that it could somehow be ok for us to still be a thing, if it included her, all the better!
But no. He was ending our friendship, even, because he had met "the one."
The whole thing just saddened me.
Then I was married a long while. I was monogamous, but...that relationship was so abusive on so many levels. And I was not always the victim...sometimes, I think, I was abusive to him, too. It was just completely toxic in a lot of ways. But I often told him, because he was jealous and insecure and always wanted to poke at me and ask "how would you feel if?" and I said to him so many times, "go ahead and cheat on me! I don't give a damn what you do!" And for the most part that was true, except that I would have lorded his hypocrisy over his head if he'd done so, because he was constantly scrutinizing me for signs of inevitable moral decay (I'm female therefore, no morals, according to him.) I, on the other hand, during that marriage, had a playful crush on a female friend at one point, but didn't dare look at another man.
So yeah, that crap happened.
So after that, I didn't know what I wanted other than to not get too serious too fast, not to get trapped into anything that might turn out to be dysfunctional. Keep it easy, voluntary, make no commitments. So when a man I'd started dating said he was interested in co-dating another couple (a married one) I was like, well...why not??
Now I'm mono again, but it's just what's working for me at the moment. I'd never say that poly doesn't or can't work...just maybe I don't have the time, energy, bandwidth at this time in life.
So I don't feel I had any sort of a major life decision to BE POLY FOREVER. Do what works for you, when and as it works for you. I don't believe this sort of thing HAS to be a lifetime declaration of personal identity, although for some it certainly is...and that's fine, too. Too fast, too slow...who cares? You do you.