I agree that it can depend on social circles.
Still, I would argue this:
Lets say your 19 year old daughter wants to introduce her boyfriend of same age into your family.
How would you rate her (and his!) seriousness if
a) they lived in his apartment or
b) they are organizing their wedding.
I would wager b) would be "well-established" much quicker, especially in the wider circles, and thus have more "social resources" sooner, thus giving their potential offspring better chances. Of course, with kids out of the picture, it wouldn't make much difference. But that is exactly my point: Marriage serves having children.
Still, I would argue this:
Lets say your 19 year old daughter wants to introduce her boyfriend of same age into your family.
How would you rate her (and his!) seriousness if
a) they lived in his apartment or
b) they are organizing their wedding.
I would wager b) would be "well-established" much quicker, especially in the wider circles, and thus have more "social resources" sooner, thus giving their potential offspring better chances. Of course, with kids out of the picture, it wouldn't make much difference. But that is exactly my point: Marriage serves having children.