I find this discussion about marriage kinda odd. You are saying that if two people simply agree and do some kind of ceremony (before God) they are officially married, yet do not have to be bound by a legal agreement. I find that very strange. It's like : " God is my witness" and no accountability whatsoever beside that.
The ' legal' marriage will allow you to have a married status in all kinds of different situations. when a spouse fall sick, or in hospital, the person you are married to gets contacted, not the person only you, and God know you got married too.
There are a lot more examples I could come up with to show that it is necessary to be bound by a legal agreement for the society to acknowledge the marriage. You can say: I don't care about the world to acknowledge, but there is supposed to be accountability, right??
Ive heard many stories about african men seducing women into marrieing them this way, they only do a ceremony in church and they say its is valid. When it comes down to staying permits, birth certificates, etc. then the story becomes a different one. I ve heard over and over again that the so called marriage; was no longer respected whenever trouble arrived.
I was in a relationship with someone long ago, we were together for a long time, we lived together etc. We were not ' officially' married, but for us both we were going to be together for life. Yet he fell sick and died, and during that process , even though I took care of him, I was in no way considered during his sickbed, death, and funeral. Nor did I get any of his inheritance or things. That was a eye opener to me. It is important to have a ' valid' marriage, simply because the world will treat you as married. And if you dont, they won't.
Tit 3:1 Put them in mind to be subject to principalities and powers, to obey magistrates, to be ready to every good work,