Well, sorta-controllable. The ability for men to control their physical and emotional arousal is a skill of varying strength, kinda like raising one eyebrow or peeing without getting it on the floor. Men who can control laughing at Mitch Hedberg are more able to avoid sporting an uncontrollable woody, while men who can't help laugh at stoners get erections unabated by will. The Incredible Hulk, apparently, should have been drawn with a ten-foot-long penis at all times, but this does explain why Dr. Manhattan was consistently flaccid.