If I have time, I explain how much I'm loving ever single minute of being with my son and how amazing he is, unlike anything I ever could have expected, and I can't begin to think about having another child while I want to spend every free second eating up my amazing little boy.
If I don't have time or don't care about the person, I tell them the other truth. I can barely afford this kid. There's no way on earth I could afford a second one.
There will always be a question to ask people, because society is never happy with the now. It's a shame, but we will always have to come up with excuses regarding why we want to live the live we so choose to live.