> EDIT: Turns out this is NOT true, or at least, nothing I can find seems to point to it being true. Just habituating with someone for a long time is not enough.

It isn't true in California, it appears. It is true in a number of other places worldwide, Australia in particular.

Even in Australia, a de facto relationship – in which partners have a right to a property settlement upon separation – requires more than mere cohabitation of sexual/romantic partners. However, in practice, relatively few long-term cohabitating partners will be found not to be in a de facto relationship.