If you marry young you can have 12 children by the time you are 30 and even grandchildren. In the days before birth control via pill that was a reality.
as for DINKS the ones I know are infertile or have health issues that stops them having children or married really late. My mum said she had her tubes tied after the birth of my younger brothers, they were twins. (four in my family) I don't know about the marital lives of my aunties and uncles who stopped at two or three children, I don't really ask them that question!
I do think that some married couples just stop having relations at some point but its a mutual decision. I mean you married, you would be discussing this and not be naive about how babies get started lol.