Premature ejaculation (PE) is when you have an orgasm before intercourse or less than a minute after you start. There is no set time when a man should ejaculate during sex, but if you ejaculate and lose your erection this soon, you and your partner may feel there’s not enough time to enjoy it. The cause is not really known. But your brain chemistry could be at least partly the reason. Men who have low levels of the chemical serotonin in their brains tend to take a shorter time to ejaculate.