Excellent question.
First to clarify: the Society does not make negative statements about having children--they are very neutral about this. This has changed since the 1975 age when child bearing was discouraged.
Also, most JW couples I know are fully aware of what "make a baby". But with recent changes about vascetomies (Society discourages, no "priviledges" if man has operation), JWs are forced to use condoms or birth control. Since either method has its drawbacks, "accidents" are common. More commonly, children are conceived on purpose. I see this all the time, especially if there is already a first child.
My opinion is the Society is neutral on children because it is a primary territory for them. Baptisms in my area are consistently 90% JW kids between 14-18 years old.
The mind set of the average JW is hard for me to reconcile: impending Armageddon (with the implication of persecution and family seperation) versus the natural desire to have a family. Also factor in that most JW youth do not stay in the religion past their 20's--as a parent you are almost certain to experience the DFing or fade of your child.