Couples beyond their child-bearing years? The public purpose is that it channels sexuality in a positive way. It helps prevent fornication/adultery and the transmission of disease.
Additionally, it's also possible for them to adopt.
I am married no children and I have seen so many marriages where there is infidelity, emotional abuse it's ridiculous many marraiges staying together because of COMMITMENT full of lies because 2 people are unhapopy or feel there stuck this is reality, not being pessimistic just looking at the hypocrisy in marraiges.... SAD BUT TRUE.
How so? If people want to cheat, they're going to cheat. Marriage doesn't change that.