Id say the argument that we drink milk because we also do other unnatural things completely unrelated to food is also flawed.
You could use that argument to say we only drink milk for money/profit, like everything else. Not because its actually something we want to do naturally 

 I'm not saying we drink milk because we do other Unnatural things, I'm saying ruling out a behavior because it's Unnatural " is illogical  

 Its definitely unnatural, just like eating any of our other unnatural creations, you wouldnt consider coca cola healthy 

 https://en.m.wikipedia.org/wiki/Appeal_to_nature

Again, your argument lacks logic.