I spent some time yesterday looking into this question too. There looks to be a lot of people looking for the same solution. It's not even a modern convention, plenty of cultures don't have 'legal' marriages but live as spouses. Some don't even have a concept of marriage but still have relationships that are just as valid as formal marriage.
At any rate, the best solution I have seen other folks suggest is:
- Set people as spouses
- Don't set the location (this should be used to confirm formal/officiated/legal marriage/marriage-like ceremonies since these sorts of records include location)
- State the year (not exact date) the relationship started (and ended if relevant).
- Describe in the biography what their relationship status was, including any relevant details (you could include a location e.g. where people met or first lived together, etc. and exact date here).
(Apologies for the lack of a good descriptor word that's inclusive of all spouse type relationships - I can't currently think of one catch-all word in English that includes formal and informal marriage, de-facto relationships, common-law marriages, civil unions, situationship, and all other committed relationships)