This is all very interesting. I don’t have much to contribute, except to say that just last night, my SO was talking about a good friend of his, who has an open relationship with her fiance (and who, IMO, also has the hots for my SO), and I will tell you that my first response was, “Huh . . . why are they getting married, then?”
(Please note that this was not an “Eww, what freaks” judgment; It just seemed to me that if these are two people who know that monogamy is not in their plan, they could just skip the whole parade down the aisle. Also note that although I favor monogamy and have no problem committing to my SO until one of us kicks off, I feel like I can take or leave the aforementioned Veil-and-Tux parade myself.)
Now I feel like I can understand a little better how some polyamourous folks would still find meaning in having a ceremony to honor their commitment to / relationship with each other.
So thanks.
Nothing like a good education. 
As for an answer to the OP, all I can say is that whenever I’ve dated more than one fella at a time (very casually, everybody knew what was up), I invariably had a preference as to whose company I enjoyed the most. I would be out with one boy, and I’d be thinking, “I should bring [Boy#2] here for his birthday” or "[Boy#2] would love this movie . . . " Usually I’d end up wishing I were sharing the experience with Boy #2, and sometimes even count the minutes until the end of the date (which was by no means a Date From Hell), so that I could get home and call him. And that didn’t seem fair to Boy #1, because when I was with him, I wasn’t totally “with” him. And Lord knows I’d hate for anyone to be that way with me, being the narcissistic princess that I am.
So, invariably, I’d eventually end up in a monogamous involvement with the boy I preferred.
I don’t know if I’d go as far as to say that I’m just hardwired for monogamy, but I’ve never really considered polyamoury a workable option for myself.