My coming to terms with being poly fell, essentially, on three key ideas. You can evaluate and discard/modify/adopt/torch as you see fit for your needs.
Point one, monogamy is, at its core, an authoritarian relationship based on ownership of another person. When you are in a monogamous relationship with someone, they are YOUR husband, YOUR wife, YOUR boyfriend, YOUR girlfriend and they are only allowed to give their love and affection to you. If they stray outside that, they are committing infidelity which is considered a bad thing by most people. You control that other person in that respect. You are only allowed to get certain forms of personal fulfillment from your partner and going outside the bounds of that is "bad." I am fundamentally uncomfortable with that sort of relationship.
Point two, mutual happiness. If I love someone, I want them to be happy. If they are happy with someone else while still being with me, that should be a good thing. If I feel threatened by that, then perhaps I have some insecurities I need to deal with.
Point three, my own feelings. I got to a certain point in my life when I realized that people I'd loved and cared about prior to that point...I still felt that way about most of them. I realized that not only was I capable of feeling genuine love for more than one person, I actually DID. That sort of negated the "monogamy = loving one person at a time" idea.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."