Very good points. I think it caught me so off guard because I have no issue explaining it to adults. I have several medical problems that would make carrying and caring for a child very difficult. Karma and I are both very worried about the genetic background of both families so that plays a role. And then there's the fact that we are hardly supporting ourselves and we want more for our kids. Not to mention that we both raised our younger siblings and while for me that didn't really change anything, for Karma, he never had a childhood so now he's living that out.
But for whatever reason, a simplistic answer just didn't come to mind. My usual quick response is that we like our time to be freely spent how we choose. Again not an answer for a child. I told her that if we had kids we wouldn't be able to come visit her as often as we do and that we like being able to spend time with her and her brother. She seemed satisfied with that.
I've never really had to defend my answer. My parents have always supported my choice and Karma's dad was a little upset by not having grandkids but he's slowly come around. Other than that I don't really defend my choice as much as use the question of why as an opportunity to educate.