My son is an only child and for years we have had relatives and neighbours telling us that we were somehow depriving him. He is seven years old and I am now expecting another child. The reason for the huge age gap is so that we can actually afford the necessary childcare.
I know other families with only children in real life and they also get insensitive comments. For example my son's best friend is an only child because his baby sister died at birth. His parents are just not emotionally ready to think of having another child. Why should they have to explain their reasons to nosey parkers. Their little boy is one of the nicest children I know. There is no evidenece that he is being harmed by having no siblings.
Reasons for family size are intensely personal and I wish that people would just mind their own business.