some questions you might want to ponder alongside this:
what does it mean to be "natural" (in this context)?
is there some set of behavior that is universally natural? (even if just referring to humans, and not all of life)
is there a single, objective way of perceiving and interpreting human behavior?
how did humans exist on this planet for many hundreds of thousands of years prior to the rise of civilization and the nation-state?
do you think there is a single, objectively correct way for all of humanity to live?
The heart of the matter is that human differences are real, and generally greater than most defenders of hierarchy will acknowledge, but the universal yardstick by which any evaluation of those differences might be considered natural and definitive does not seem to exist outside of the various systems that have been imposed on us.
1. There is no such thing as 'anarcho-capitalism.' There is nothing anarchic within capitalism. There is just capitalism, which is the accumulation of capital which has never, nor can be, accumulated through anarchic means, since...
2. It is impossible for capitalism to exist without the state or without being the state. Capitalism has always relied upon statist means in order to accumulate capital: through enclosure; through colonialism and social fragmentation; through slavery; through genocide; through compulsory education; through the state-sanctioned churches . Always. Capitalism has always been reliant upon gunboat diplomacy and/or followed in the wake of the complete shattering of indigenous lifeways through such 'diplomacy.' Always.
3. Your interlocutor (as you present them anyway) unquestioningly presumes: a) mass society = all, or the high point of human socializing; b) civilization and history at the expense of pre- and un-civ and so-called 'pre-history' which has not always included hierarchic/class relations (nor has it been the oft-repeated yet demonstrably false assumption of the Hobbesian 'state of nature') c) they have a clear idea of what 'nature,' and 'society' means; d) that one must think in terms of false dichotomies such as, leftism or capitalism; e) unquestioningly believe in industrialism and progress, mass society, some kind of mass production, etc.
You may wish to mine the following questions for gems to cobble together a more thorough response:
"I used to be one for a while before I made the jump to genuine anarchism. That's how I knew he was wrong."
just a thought: i'd be careful of claiming your (or anyone's) ideas as "genuine anarchism". it starts to sound like "the one true way", which is really no different than any other religion or dogma.