Marriage is as obsolete as muscles, and fertility clues (birthing hips, nice boobs, youthfulness, etc.). Muscles are obsolete because we have tools; muscles use to be a sign that you could protect yourself and your future family. You could provide for them -- hunting, farming, etc. All of these things required being healthy and fit (basically having muscle). However, with the advent of modern tools, those things can be provided to a girl w/o having muscle -- commercial farming, modern manufacturing, firearms, etc.
Same thing with fertility clues. Women can now get pregnant a little bit later in life with modern science and less likely to die during child birth. Fertility clues are not as important -- even if men value it a lot.
In the past, life was harsh. Many people died of starvation if they couldn't work... if they weren't able to farm, hunt, etc. People need marriage to work together as a team to raise children and provide for them. And to protect themselves from the elements, to fight for scarce resource, etc.
Men in the past help created our current society where life is easy and poor people die not of starvation, but because poor people eat too much (heart attacks, obesity, etc.. etc.); they die from over eating. Men in the past have done such a good job creating a "good society" that they have made marriage obsolete.
Rationally there is no reason to get married. It's only our lizard brains that want us to get married. The same lizard brain the find fertility clues "hot"... or men with big muscles "hot".