Every person I know, in their 20's, could have written the same post as OP... Myself included. Now they are all married with children... Myself included.
Really? As a person in my 20s, I have a reasonable number of friends who have plans of getting a house, spouse and children; many have had them since their teens.
Of course, nowadays those plans are less taken for granted, considering many can't even get jobs.
In my 20's, most of my friends and I were in one touring rock and roll band or another. A bunch of societal malcontents, dropping out of college and trying to do something else. Maybe that's not terribly common, but I really didn't know many people who pined for the family life... but biological urges are hard to fight... Not just the urge to procreate, but the urge to raise and nurture kids. The urge to nest and create stability. It's not for everyone, but it IS for most people. Not because of some societal constraint, but because of biological ones. Societal constructs just helps us to fulfill those desires more easily.
Of course, nowadays those plans are less taken for granted, considering many can't even get jobs.