Or they are just barbaric and want to cause violence to women and children
I highly doubt that a rapist (outside of war) is trying to just pass his genes along. Hes committing an act of violence just as is done when at war.
A rapist is never consciously trying to pass on genes.
A rapist is driven by lust. Lust is the instinctive need to procreate.
In civilized societies, we disdain uncontrolled lust and try to contain rape, even in war time.