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.
Did you miss where I said in my post my husband and I suffer from infertility? I assure you I know more intimately about both adoption and advanced reproductive technology than most people (and by the way, it's been a heartbreaking several years, so I'd appreciate it, if you care to continue this discussion, that you spare me the sarcasm).
I've learned through talking to many others afflicted with infertility that there are some people for whom fertility technology cannot work. There are also many reasons that preclude some people from adopting. It's a long, messy, bureaucratic process, and not everyone gets approved. So how do you propose to deal with the sad group of people who want children, but have exhausted all avenues in vain? Will you de-marry then? If not, how do you justify not doing so?