I'll chime in on this one.
I grew up in a very traditional family structure. Both of my parents were heavily involved in the Lutheran church (father almost became a Pastor). I also watched (from a very young age) how lies, cheating, and divorces can tear families apart.
My brother went through a divorce after having two children with his (now) ex-wife. My uncle went through a divorce after having three children. My cousin got a divorce after cheating on his wife 3-months after the wedding. And my parents would have gotten a divorce (my dad had an affair) if it weren't for the fact that my mom knew that something was wrong with my father (we found out later that he actually had a brain tumor that affected his personality and caused him to do things completely opposite of his nature).
Knowing how many marriages end in divorce, I've never wanted to get married, but I always wanted a loving, monogamous relationship. My dad's eventual death and the turmoil his affair caused within my family, and also seeing how my brothers divorce affected him and his family, resulted in me often being left to figure things out on my own with very little parental guidance (due to all the drama happening in my family). I grew up with a lot of trouble talking to women, a lot of failed attempts at relationships (mostly from being emotionally unstable myself), but always wanting a relationship because I knew I would be good at it.
Fast-forward to 3 years ago when I met my currently girlfriend (she's basically my wife). Our relationship is the longest I've ever had and the most stable one either of us have enjoyed. We both love each other and have a child on the way (due in 6 weeks in fact). I actually never really considered polyamory until about a month after our child was conceived. I always thought of myself as mono until the moment I met this amazing girl who I'm totally in love with right now. We connect on so many levels and I can be totally open with her. I connect with this girl in many ways that I don't with my wife (we have very similar spiritual beliefs, whereas my wife is Agnostic / Atheist). My wife is my rock, but this new girl is like a pair of wings. She makes me feel like I can fly. They both bring something very special to the table and I have no interest in leaving my current relationship for another one that may or may not work out.
So I'm pretty much in the situation that InquiringOne defined in his original post. I'm "married" with a child on the way and in a position to lie to my wife and cheat on her with this new girl. Knowing how affairs and lies and poor communication have negatively affected other members of my family, I wasn't going to have any of that.
So one night, a few weeks ago, my wife and I had a conversation about "Love" and what that word means to each of us. My definition of love hinges on "trust" and "freedom". To me, real love cannot exist without trust, and trust requires honesty, openness, and good communication. The conversation evolved from there and I began to explain to her that:
- I believe it's unrealistic to expect two people to only love each other for the rest of their lives;
- I believe that it's possible to love more than one person at the same time without one love taking away from the other (after all, you don't love your first child less when the second child is born, so why would you love your current lover less when a second one comes along?);
- Marriage and monogamous relationships have a long history of being about "ownership." Marriage for most of our history has been about a husband "owning" a wife. I don't believe relationships are about ownership of the other person's love, but about allowing your partner to have the freedom to express his/herself (and his/her love) freely;
- Society is screwed up and wrong about almost everything. We live in a world where it's perfectly acceptable to hate a person for loving someone else, but it's unacceptable to love two people at the same time. The world needs way more love and much less hate;
- Cheating is fundamentally about a betrayal of trust (and thus love) not about the actual sex act;
- It's perfectly natural to be attracted to other people and I believe that forcing your partner to abandon what could become a loving relationship only breeds negative feelings.
- I want to be happy and I want my wife to be happy. Which means I want her to be free to pursue other relationships if they come up, and I would like the same freedom;
- Knowing that my partner will allow me to express my love for other people outside of our relationship only makes me love her more and want to keep her in my life;
- If we do pursue other relationships on the side, our child has to be our #1 priority and the other partners need to be good influences on our children.
I said more, but ultimately I laid it out rationally and logically and my wife agreed with everything I've said. It also helps that she's not hung up on sex like a lot of people are in society, so she's much more open to the idea of polyamory and open relationships.