I have always thought that the Oath Rod stops the physical ability to have children. It was originally used to punish criminals, who the AOL AS might not want to have children in their perfect society.

Secondly, the tightening effect that comes over them when they swear seems to "freeze" their current faces, so that when they get older their faces seem ageless. Rand mentions thinking Moiraine looks young one moment and older the next. Women channelers who aren't bound on the other hand just look young. My theory is that the tightening freezes the rest of their bodies in time, too, and they just can't have children.

It makes no sense, and is completely unfounded by any evidence to be found in the books, but this is how I've always explained it to myself.