Delayed ejaculation, difficulty achieving orgasm during partner sex, seems to be much more common than it used to be. The skin of the penis is constantly exposed to friction of some kind or another. Even when men are not active, clothing can chafe and rub the skin. This repeated friction causes a toughened layer of skin to build up, like the calluses that develop on the palms of the hand. Because of this, the nerve endings beneath the skin of the penis are less able to experience sexual stimulation, leading to decreased sensation.