Marriage

 To me it is very simple.

Marriage is a umion to begat children between a male and female when biologically possible, based on hetrosexual love. Even if the sexes marry for another reason, as male and female, and prefer childlessness, the possibility of the better remains. But, as they decide on a union of selfs, they deny society a future.

Because without children born of that society, it has no  future in the end.