Well... if it's a story that takes place on Earth, you consider the time period, geographical location, etc. Dudes in pre-Meiji Japan? Foreskins! Dudes in modern America? Probably not!

And in non-Earth stories, you have to think, well, is there anything in the surrounding culture that would make the people of this world decide to cut bits off of their penises? Or not do so?

AND IN THE END, IT'S ALL FOR THE PURPOSE OF PORNO :D