In my new family, I purposely adopted someone that said "No Way!" to having kids just to see what would happen, and she and her "Not Really" husband went on to having five kids! They made a baby on the first try every time I put them together to make babies.
I have noticed that it kind of seems like I get more chances to adopt kids when both parents are "Definitely" or "Would Be Nice", but that could just be a coincidence (or my mind playing tricks on me, he he).