Here's an article talking about a legal arraingement in France some 600 years ago that was used to formalize a family relationship between people who were either related or unrelated, making them a household. It could also have involved same sex romantic couples.
Of couse, anyone who hates the idea of same-sex romantic relationships having any legal standing will either claim the research has been faked, or simply dimiss it as French.
Personally, I don't find it surprising. We know for a fact that other cultures had social/political/civil structures that accomidated such relationships. And to be sure, there are non-sexual reasons why people who are not married might want or need to form a household...especially in such time when society had fewer built-in stabalizing forces such as we have now.