It seems to me that the inventor of the button-fly jean was a person who spends the winter in a villa somewhere on the Mediterranean, or at least a trailer park in Florida.
The reason for my supposition is heat pipes.
On the three different pairs of button-fly jeans I own, the buttons are stainless steel, and there is no denim flap behind the fastener behind the button backs, as there is with the zipper-fly models behind the zipper. The typical denim flap of the zipper models is in fact what the buttons are mounted in.
The problem arises when a) the button backs press against the wearer’s body (or against the thin fabric of the usual undergarment), specifically against the part of the male anatomy that is typically buttressed against the fly of jeans, and b) the button fronts brace themselves against the Canadian winter.
Those readers who are familiar with the mechanism of heat pipes will understand why I am now wearing long johns most days on my walk to work.