Generation after generation? Where exactly do the new generations come from if they don't breed? Do they snatch babies from orphanages or something? Quite an example of solving a problem by creating another problem. If couples found the ninja style of life too stiff and thus not worth it, why not change their methods and living environment? If both the husband and the wife are ninja, there's no way they wouldn't understand what being a ninja requires, so if the rules were reasonable, they would cope with it. However, if the lives of the couples were restricted for no sensible reason, of course they would rebel.