You are defining rape in such a way that non humans can not do it, then using the fact that non humans can't rape as an argument for a natural law against rape. That doesn't hold up logically.
Sexual coercion, including forced intercourse, exists in animals and in humans. There is no reason to think that sexual assault is wrong from a purely "natural" standpoint.
This is getting away from the topic at hand though, do you have logic for a "natural" law against abortion?