do most people with zip fly button their button first, and then zip up? if so, you could even go so far as to say that the downward "away from your body" motion necessary to grab at the zipper is, in a body language sense, too extroverted to be conducted around one's genitalia. a sequence of several fastenings that protect your genitals, beginning away from your core and proceeding toward it, is more assuring and, as a result, habit-forming. we could even do an experiment... too bad Imma use our grant money to buy some 3sixteens fuckyeah. I think the major thing at play is the fact about the ease of repeating a motion several times compared to the difficulty of starting and stopping two different motor programmes in sequence. many buttons may simply be more 'automatic' and require less conscious control.