Dark Age Christianity didn't preach equality and democracy: it taught that submitting to those above you was God's will, and ruling those below you harshly was good for their souls. In Northern Europe it imposed hierarchical values on a society that had been egalitarian and individualistic, and pulled women down from being more or less equal. Not exactly the coming of fluffy liberal ways.
That's their opinion. Others argue that the natural state of humans, like our close primate relatives, is co-operation, with dominance used only as a minor social tool. The widespread rule of warlords and tough males is an aberation from which our species has begun to recover. (Right now the recovery doesn't look so good, but we can hope.)[color]
The "Transvaluation of Values" mentioned by Nietzsche and the "Bio-Truths of Dr. Lange (John Norman); as well as many other philosophers, consistantly point out how modern political and social values have been twisted to emasculate mankind, which does a great dis-service to both man and woman.
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We can enjoy and benefit from playing with the leftover instincts for peck-order living, just as we play with the instincts for hunting and fighting, but it's neither necessary nor healthy to build society around them.
If D/s arises because some are "born to rule" and others have an inborn need to submit, where does that leave switches like me?