People comment on it for the same reason they say stupid things about any number or arrangement of children in a family. Many folks just tend to run at the mouth.
I have one DD and one DS and people comment all the time.. I get comments that assume that we're DONE because we have a "perfect" family. Uhm, not true at all. We plan on having 3-4 kids, but our society says that 2 is perfect (especially one boy and one girl) and that we should STOP! Do what works for your family and ignore comments, people simply blab too much.