It honestly depends on the couple.
I have never wanted to be married. My parents were and are married but growing up I was raised to not feel like I needed that to "complete me", not settling for someone (someone like a lot of husbands you read about on here) to get a wedding, not relying on another person financially.
I know I will be with my partner forever. He would get married if I said I wanted that, I just don't feel that I need it. We both have great careers, we both contribute equally, we parent as a team. I didn't cut down my hours after maternity leave and continued to pay the same into my (excellent) pension scheme.
We are obsessed with each other, I like that our children will see how much we love each other and how we choose to be together. Every day we put our relationship at the top of the priority list. Sometimes I don't know how we found each other.
I honestly do not feel that I need a legal document to trust that he is stuck with me. I do get it's different for everyone though, I know some people prefer to have the security of a marriage and if you are a SAHM it is very important.
I know lots of couples who are either getting divorced, separated or deeply unhappy together. I also know a few who are happy. We are the only couple who are unmarried of our friends.
If one day he decided to up and leave me, I would be absolutely devastated but financially I would be more than fine. I also wouldn't need to take from his pension later on in life as mine is excellent.