Well, it might help to recognize that for a lot of people who do that, it isn't painful, or it is a kind of discomfort, for lack of a better word, they enjoy.
Pain and pleasure are on a continuum, and are very individual both physically and psychologically. So, while getting or having a genital piercing might be painful for one person, it's pleasurable for another.
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead