It's their delicious hypocrisy. On a primal subconscious level, femininity is about being removed or divorced from the physical requirements of reality: the woman sits back at the cave being sweet and sexy and raising children, focusing on the more abstract social aspects of life. Meanwhile Grug ventures out and hunts and takes by force. He may be hated for being rough and mean and always gone, but Grug knows that without him none of the woman's society would exist.