It's not.
If your objective is to tie marriage to children in all cases (to the extent that the elderly couldn't marry), how do you justify allowing younger people who can't have children to get or stay married?
Ever heard of adoption? How about IVF? ICSI? If your intent is to raise children, there is no reason to believe that it can't be accommodated, especially when the supply of adoptable children under the policy would increase rather markedly. The point is for children to have parents. It's that important.