Not sure about "everyone" all marrying everyone else once they turn 18, but i can see this in a compound. 
did that until it was made illegal. Charles Manson 
led the "Manson Family," had a sex-filled quasi-commune in the 1960s that almost did that and David Koresh and his Branch Davidian religious sect, did something close to that. but maybe it would be more realistic to have one main guy that is a fast talker convince some people that he knows from somewhere to try it (where all adults are married together). I know, in the right way and in the right mood I might even give it a try. I know I'm weird but I always liked polygamy only this way is even better. I can see how women that think like me might go along with it just to stay connected or to feel what it is all like. 
Free love sounds great.
Me??? Heck no I am with an incredibly hunky loving guy who is almost hung like a horse. He's not as long but quite long enough thank you incredibly much.
Besides, he has a pretty good sized sex drive and can last for hours. Just out of personal safety I flirt madly with him so I almost always get a before sleep screwing.
Though I do flirt outrageously with all of the ladies at Lit (some more than others, of course).
Just an example.
Would Your wife get mad if i asked You to marry me?
Sadly, yes. I mentioned the idea of polygamy some time in the past, but she isn't open to that.
And alpha sub.
Prolly for the best, anyway. i'd want to be first wife if ever i enter a poly relationship.
Are we talking a "right-after" story, when they're setting the system up, or a "years-later" story, when it's been in existentce long enough for it to have become the accepted norm?I think that the post-apoc scenario is best. Radical solutions would be considered more quickly in such a case. What you said about sex drives is another factor...but with more than one of each sex...hopefully, everyone could find a partner when required. Wife 2 wants round 2, but hubby 1 is now with wife 1, so she finds hubby 2.
Wouldn't there also be a couple of lesbians, though?I envision there being a slightly larger number of men than women, but a few of those men are gay. Which is another choice if all of the wives are occupied.
A balanced group would avoid all this. The same number of women as men, and a "who sleeps with whom" rota so monogamous pairs don't build up accidentally. This would be essential in a "just started" situation.
Fast talkers who are convincing usually end up with the lion's share of the women. It is a natural result of an Alpha Male in a group of animals/humans.Roamme said:
