I chose to be in an open marriage. I always thought i would be monogamous. I never had any mindset other than that. After many years of marriage, and finding that neither of our needs were getting met by the relationship, we decided to open up and try to meet some needs at home and others outside the relationship. This works more less for us.
It's complex though. When I'm really into someone, I do not look at other guys or fantasize about other guys or want them. That would make me sort of monogamous. However, I don't believe that this kind of interest (coupled with the stress of a home life can survive). I can only speak for myself, but there are many many unhappy married people.
That said, I also was never one to keep myself in a nuclear family. I have had friends (platonic ones) who I would say I am as dedicated to as my family. These relationships have sometimes included daily interactions for plus 10 years. Is sex the only element of monogamy? I would say that these friendships with people I have intense emotional dialog and commitment to would also be a form of polyamory.
I actually consider what I do closest to something gay male couples have always done.