It depends entirely on what I am wearing and sometimes on how badly I need to go. Also where I am plays a factor as well sometimes.
If I am wearing pants with a belt then I am almost always also wearing boxers which means that I will just unzip and use the opening as intended.
If I am wearing something looser fitting and no belt then I will usually have boxer briefs on and will pull them down and go over the top.
If I am for some reason wearing tighter pants, a belt, and boxer briefs there is a 50/50 chance of me using the fly rather then unbuckling the belt, unbuttoning, unziping, then pulling the boxerbriefs down. All depends where I am (home, urinal, ect...) and how badly I need to go.
A suit or dress shirt that is tucked in though will mean using the fly 100% of the time. Takes way to much time and effort to get yourself all put back together and presentable otherwise usually.