I will agree before anyone feels compelled to point out that all women have an agenda. So do all men. But agendas vary.
Here is why the "good girls", the really marriageable girls, get married. These are women with giving agendas:
1. Family centric agenda:
2. Life partner agenda:
True, all people have an agenda. It has been asked here why a man would get married. For me, it would mostly be to have children. It is the best option for men who want children. Trying to have a child out of wedlock is a quick way to weekend dad, seeing your kid call someone else dad, watching your kid move away, and paying child support. At least with marriage, you have the potential to start with a good foundation for an intact full family, and you have a better chance at obtaining custody if it's better for both you and your child.
I don't see how marriage is necessary to a woman in regards to the family centric agenda, however. She births the children and unless she's a total scumbag then she has full rights, custody, and child support if she enforces it. And both genders don't need marriage to create a family and live happily ever after together. A man can still financially float the mother of his children without getting married. And some do. So it really comes down to protecting one's interests. With marriage she can add alimony to the child support, in addition to other assets he may have accumulated, if the relationship ends.
And for the life partner agenda, marriage is not necessary either. You don't need to be married to care for each other until death do you part, though there are financially advantageous laws for married couples. Yet with that comes the risk that most often falls on the man. So the way I see it is that marriage can be a really great thing, but it comes with a lot of risk, mostly for men.
Is marriage really necessary to live happily ever after with someone? No. So that brings us to the question that started this forum. Why do something that isn't necessary? I just see it more necessary for the man if he wants to have a family. And I see it more necessary for the woman if she wants a better plan to cash in on a failed relationship.