"We don't have to be married and having kids by the time we're twenty, before the plague or dysentery gets us."

There are many choices for life, and choosing never to get married and never to have kids is exactly the right choice for many people. People don't need to build an army of labor to tend the farm or whatever anymore.

Having said that, the human body is geared to having children young. A lot of women are putting off having children and then finding it either impossible, or learning that the risks of an endless array of defects goes up quite dramatically.