I first read Stranger In A Strange Land when I was very young. I think, now, that my parents, at least my dad, viewed it as a bit of primer for how life should be. [I recently found out about their attempt at a triad/vee/whatever. Mind-blowing. I knew the woman when I was a child, I had no idea what the adults were up to.]
My theory is that because I was so young, I had no idea it was odd. My life was already a grand series of odd, so I didn't really notice odd. Marry extra people? Surewhynot.
At twelve, I got to see the Harrad Experiment movie. Probably read the book after (though I don't recall exactly). When I got to senior year in high school, the discovery that I liked girls was the biggest news. I must have entertained notions of 'one of each,' though I don't specifically remember thinking that. First college boy I dated turned out to be bi, and I had a scrumptious morning once, when we both woke up and watched his roommate crush a can (from bed) across (three feet?) from us, and we both shivered with delight. I guess that would have been my first moment of compersion.
A few years later, a crazy woman tried to seduce my girlfriend and I, unbeknownst to each of us. When we found out, we told her to beat it; but she was not easily dissuaded. For a minute, we attempted to be three. We had a lovely makeout session one evening on the couch, and then they got up to go to the bedroom. 'what about me?' 'you can wait out here.' I DON'T THINK SO TIM. So that was over.
Twenty years later (10 years ago) I met my CBF, and you can find the rest of the story in my blog (I'm not averse to typing it a gazillion times, I'm just tired at the moment)
Both of them said to me, more than once, that they didn't think they could do this with anyone but us. They were very good friends and much closer to each other than they were to me, in some ways.
I don't think it's so much of a burning need for me. I tend to fall for quiet, often cranky, introverts. I am quiet, but an extravert. I can be happier with more than one ~ I have a lot to give, and spreading it around seems to ease the burden on any one person to receive it.
It's much more potential for me. It's a possibility, rather than a mandate.