Ah, I see where I stumbled a bit. What I should have said, to avoid defining a word with itself, is that you are married when you have decided to partner romantically with someone and become a single social unit permanently.
I meant “you” in the plural sense, as in “you, together” — when a couple makes the commitment to marriage, that is all that is required. Young people, teenagers included, often marry and divorce soon after, the “certification” does little to change that course of events. Divorced people sometimes reconcile, again, paper and ceremonies do nothing to change that. If the couple, together, have decided to be married, made the commitment to be married, and present as married, it makes little difference what outsiders to that relationship say or what ceremonies you’ve had or not had.
I personally think having a state institution of marriage is silly and should be done away with altogether, leaving marriage as merely a social/spiritual/community thing. This way, if you want the one person who makes medical choices for you to be different from the one person who benefits from your health/life insurance and different from the one person who you file taxes with, it could be permitted.
These sorts of things should all be separated from marriage, particularly because of what you say about relationships being messy. (If you want to write the same name on all those forms, that’s fine, but if you don’t, that should be fine too.) If you want to write a contract protecting someone’s interests in the event of breaking up, that should be an option, not essential or assumed. All these things marriage imparts ought to be optional.
It’d handily solve the problem of who is allowed to marry whom, how many spouses one can have. It’d eliminate a lot of divorce problems (though certainly not all, I suspect those lawyers would still have plenty of business), and give legal and social options to those whose relationships don’t quite fit the bill for marriage but are close in some other way.
There would be no more arguing about whether it’s OK to have your ceremony six months after your legal wedding because the practicalities would be untangled from the romantic and the spiritual, as they should be.
The point was not that marriage is so flighty, it’s that it’s a determination that is entirely up to the partners who create it, it shouldn’t be up to some outside interest to go “OK, you’re married” or “You’re not married/can’t be married.” (unless the couple invites that person, like a religous leader if they wish, to help them with their determination.)