The good news is that there’s an article about this nifty medieval French legal arrangement called affrèrement — “brotherment”. It allowed brothers, relatives, or even unrelated persons to pool their goods and share a household on an equal basis, instead of designating one person as the owner and master of the household and everybody else as a dependent. It was most helpful for several people inheriting a single house and farm, since it permitted them to get use of their inheritance without having to cut up the farm into smaller pieces or sell; and it preserved both wealth and the family homestead. But since several people can afford a bigger place than one, and since there are always economies of volume when several people live under one roof, you can definitely see the value of pooling resources with a friend or farming businesspartner to buy land and a single house. It even represents a sort of tiny corporate model, without having to worry about tangling with guilds. You might also compare it with founding a monastery, or writing out the terms of service on a ship or in a merchant venture company — particularly apropos to a custom which began in the 1500′s. (Except you get equal shares in the company — share and share alike! So affrerement was a pretty good deal.)
There was a certain danger. The worldly goods of everyone involved became joint goods, which none of them could dispose of without consent of all the others. They also became each others’ heirs, which guaranteed some fun when wives and children, or other relatives, came into the mix. (Did we mention that France had lots of lawyers and courts from fairly early in the Middle Ages?)
But is this what the article is interested in?
No, of course not. It wants to claim that this was used as a sort of homosexual civil union. Although they admit that there is absolutely no evidence for this interpretation, they claim this lack almost as a proof. Argumentum ex nihilo, I guess. They do note that affrèrement agreements shared similarities with marriage contracts; but they don’t mention that French marriage contracts even down to this century were a lot like corporate merger contracts. (Notoriously so. The English were always mocking this in novels, even though they had their own mysterious “settlements” that they were too refined to mention.)
One rolls one’s eyes and passes on.
(One also shudders to think of this gentleman’s persona story, did he join the SCA. No doubt he would have visited Tenochtitlan and there invented the Reese’s Cup. After all, they had peanuts in Mexico as well as chocolate, and besides, there’s no evidence that Reese’s Cups didn’t exist in period, right?)
Anyway, the other nifty thing is that the good frères, natural or adopted by choice, swore to share “one bread, one wine, and one purse”. Sounds like the Three Musketeers. 
UPDATE: Sherman Logan saith, “A considerably more logical explanation is that, in a society structured by kin-groups, those without relatives did what they could to invent artificial kindred.”
Also, the Orthodox are bringing up adelphopoiesis (“brother-making”), which has also been described (stupidly) as a form of civil union, when in reality it’s more like blood-brotherhood or becoming someone’s anama-chara (mutual spiritual director or confessor, aka “soul-friend”). Apparently it still goes on today. One poster recalled knowing a businessman and monk who became such brothers; the businessman stayed in the world, donated money to the monastery to support his new “brother”, and came to visit him. You can also read this First Things article on adelphopoiesis, by a woman who was made a sister.
