Marriage has a definition in our culture (and most culture). This is a culturally and legally regulated institution.
When I got married, I (with my partner) went to City Hall to apply for a license. We had to provide documents and answer legal questions under penalty of perjury. I don't remember exactly what was asked... but I do remember laughing at some of the questions (regarding whether we were cousins etc.).
I was having sex with this woman long before the legal process. The brain chemistry had already taken root, we were emotionally bonded (at that point at least). Yet we did not consider ourselves married until after completing the legal process (nor did anyone else).
Our society regulates marriage. We have rules about who can marry whom. We have laws about the obligations of married people, and the rights of married people.
Later in life... when I went through the legal process of divorce, I saw this at work.