Ever since an early age (I mean pre-puberty) the idea of choosing "one and only, forever and ever" made little sense.
I remember reading a story about a very limited open relationship when I was about 12, and thinking, gosh, why don't they open it up MORE instead of running away at first sign of difficulty?
(The plot was that this couple had "base rules" that they could have sex with anyone they wanted, but ONLY ONCE, and NO FEELINGS ALLOWED. You can guess what happened.)
Later, I just came to love more than one person, and after struggling with trying to make sense of it, I decided that I didn't want to choose, and I would do whatever it takes to make it work somehow.
I started reading all sorts of books about making relationships work, and applying what I learned. I can't say it was easy, but it was much less painful and more rewarding than what to me seems like cutting off an arm because you can't be bothered to learn how to use it in coordination with your other arm.
Later, I found out that my grandparents actually had somewhat of an open relationship, although it was obviously much more difficult back then and in a conservative society, so perhaps it is just in my genes?