I've been wondering, and this is probably a good idea for a story, about some dumb shit someone once said, regarding heteronormative monogamy ... Basically, it was about how people aren't really designed to live as couples, but as threes (or more).

Can you just imagine? If it took three to get married and buy a house and raise kids? What a remarkable population control that would be. Think about how impossibly long, if ever, one needs to find a mate in 'normal' circumstances - be it straight, gay, fluid, whatever. It takes A LONG time to find a companion ideal to your needs and expectations. Ye, sure, teenagers mate all over the place and that's probably what keeps the species going, but consider having to fit another piece into an already complicated situation. The odds of BOTH liking the third mate, of 'happily ever after for THREE people, when odds of it for two are grievously unfavorable. 

It would be awesome. I can't wait to watch that go down :D