Interesting concept, sure. The whole thing presumes there is a correct number of humans to have and that it is >= the current number. I don't make that assumption and don't see any reason I should. Change the number up or down by force and we're enemies. People just individually choosing not to have kids doesn't really have any moral value. 

 What are you referring to by changing that number by force? 

 Rape, murder, taking away people's options to control their own destiny and body.