You know, this is an excellent question and can stir a great debate. In fact MIT did some studies on this and they found that 48.7 % of people actually zip up pants before buttoning while 51.3% buttoned, then zipped. Many and various reasons were given for both preferences. It turns out that amongst men, those who were particularly proud of their endowment (whether imagined or real) were more likey to zip first, thereby lowering the possibility of getting “caught” so to speak.
Those who buttoned first were found either to have no delusions as to their gifts or were very conscientious about going about the act of buttoning then zipping.
I prefer to zip first, knowing the true measure of what I am and therefore, being conscious to preserve and safeguard that which is.