[Tamar1973]

If they're Catholic, their marriage is non-sacramental. It's even arguable whether this is a valid natural marriage.

So couples who have infertility issues don't have a "valid, natural marriage" either?!

[Aquinasfan]

So couples who have infertility issues don't have a "valid, natural marriage" either?!

The difference is intent. Infertile couples aren't infertile by choice.

There's also the category of people past the child-bearing age who get married. For some of these people, it's better for society, and for them, to get married rather than to become "dirty old men" (or women). As St. Paul said:

1 Corinthians 7:8-9

Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I am. But if they cannot control themselves, they should marry, for it is better to marry than to burn with passion.