I have heard the case made that a woman who lives in a man's house is potentially his in the larger sense. Sometimes there's a ceremony to mark the occasion and to have witnesses who can attest that the relationship was consensual. Sometimes it can be the simple fact of a man deciding that a woman who lives in his house is also his woman.
In some states and countries courts have decided that a civil union exists if people have been living together in a marriage like situation for an extended time. Would have to be years however. This causes issues when it comes to things like medical costs and who is liable.