I have a friend who has three kids and she doesn't understand why everyone else doesn't want to follow suit. Her kids are great, but they run her absolutely ragged and she's always saying how tired and worn down she is...then she wants to know if I want to join in the reindeer games?! Ummmm, no.
When I'm asked by her, and others, "don't you just want to have a cute little baby like this?" I am prone to answer with varying degrees of "no." If it's a close friend they will usually get a response of "Heck freaking no!" (using much less polite wording)
If it's polite company or someone I don't know very well, I usually say something along the lines of "No, I have plenty of nieces/nephews/friends' kids to spend time with and dote on."