Ok, it really says, "Therefore, each couple must on the average have slightly more than two children if a population is to remain stable."
I get it now. It's still worded quite poorly. The word "each" doesn't belong in there. It should say something like, "Therefore, couples must on the average have....."
Declining population is due to two factors not one: first is the that people all over the world are living longer. Second is, of course, the urbanization of the population worldwide which always includes lower birth rates.
See "The Skeptical Environmentalist", a scientific book debunking modern science myths.
I apologize if my proof reading was less than 100%--ViaVoice is good, but you still need to go over everything twice.