There are economic and legal reasons for marriage. Whether one agrees with it or not - and I don't, BTW - it is simple fact.
Not being married makes it more difficult to pass on assets to a partner. You can do it, but you had better make a valid legal document (or several, in the case of our state) that conforms to your state laws, or it's useless. Your partner, regardless of how long you've lived together, will have no legal status or rights compared to the most remote of your family members.
We knew a self-employed guy, a vet and skilled handyman, who had divorced but was on very good terms with his ex-wife. They were best friends; he even brought her over to meet us. She was a delightful woman.
One day he was in an auto accident. Severely injured, spent time in convalescent facility, then was advised he needed to be in asst. living or hire someone as caretaker as he was no longer judged competent to handle his affairs. He wanted his ex-wife to be his healthcare agent and they had drawn up the documents, including a will. We had asked them about it once and they assured us they had done all the paperwork necessary.
BUT THEY NEVER SIGNED THEM.
They thought such documents needed notarized signatures, and they just had never gotten around to doing it.
If they had only mentioned that fact to us, we could have told them that our state changed that law over a decade ago! All that was needed was signatures of any two people. My spouse and I were right there and could have signed to make his documents legally binding.
As a result, his ex-wife could only watch as his sister, with whom he had never been close to in a case of mutual dislike, came in from out of state, put him in a vet nursing home, sold off everything he owned (including his house), took all the money and went back to her home.
His ex-wife believes he could have managed just fine with a day worker along with her own help (his affairs were fairly simple). But she had no legal standing at all. Zip. Nada. None.
As for those of you who believe in common-law marriages, please do check your state laws. Our state changed its law about that decades ago, and it is no longer true. No marriage license, NO RIGHTS.
And no, I don't believe I was living in sin for the first seven years of my relationship with my spouse. Counting those seven years we've been together for 48 very happy years!