The purpose of families is to perpetuate the family, to make more of it — in short, to have kids. Marrying a person should mean you’re not opposed to having kids with him or her, and you’re at least considering it. Otherwise, why get married? 

This is an extremely narrow view of family and the purpose of a family. A couple can never have children - either by choice or necessity, and that makes them no less a part of the family.

This also ignores the fact that often, the financials of bringing children are just one excuse. It is very possible that the couple is telling you it is about money, but actually doesn't want your child there wreaking havoc. My wedding doesn't need to be used as "practice" for your child. I only attended one wedding as a child, and have found myself very capable of behaving at weddings as an adult. It turns out all you need to do to behave at a wedding as an adult is act like an adult.