Premature ejaculation occur when a man expel semen immediately with minimal penile stimulation. The question is how premature is premature in sex when some individuals enjoy quickie. So premature is a function of both party having the intercourse but according to International society of Sexual Medicine, they define premature as ejaculation less than one minute after penetration. So no matter how much you love quickie, if you don't last more than a minute in bed then it's abnormal. Maybe you're suffering from premature ejaculation. The statistic of men sufferring from it should scare you, 1 out of every 3 men between the ages of 18 - 59 suffers from it. This should give some concern and hope knowing you're not along in it. Many believe it is psychological but let's view their frame of reference and see it biologically. Although ejaculation is a reflex action controlled by the central nervous system, when the penis is trapped with blood, it causes an erection.