Why Just Girls?

I know why it has to be children, but why just girls? This I cannot determine why, but I know I it is so. Fontaine says "ah, one less bathroom to build in the orphanage." It is amazing to watch the effect of Adam on there small bodies. There own cells replaced by the new stems the instant they are damaged. These children are practically invulnerable. It is a shame you could not do the same to an adult. There would be quite a market for a man you could not kill.