For the most part, unmarried couples live together for economic necessity. In my case, both my future wife and I owned homes and one home was not being lived in. We moved in together, and put the empty house up for rent and earned some income. enough income to expand our real estate portfolio.
I think it’s for several reasons….testing the waters to see if it “works out,” quicker and easier, financial etc
But in the end, none of those matter. Getting married is about navigating those risks together.
We could also entertain the idea that it’s vastly more economical and for tax purposes quite positive to be a single mother. However, it’s detrimental to the children and future generations to have an in tact traditional family even if that means fewer tax advantages.