"Since one of the functions of pre-cum is to neutralize the acidity that urine causes in the urethra, it is safe to assume that urine does kill sperm. According to recent research, pre-cum does not contain sperm; however, it can pick-up sperm left in the urethra from a previous ejaculation but only if the man has not urinated at least once, which would kill the sperm. So if a man has not ejaculated in 24 hours, and has urinated several times (thus cleaning the urethra), chances are that his precum will not contain any sperm at all, or so little that it wouldn't represent any pregnancy risk.and that is why the withdrawal method of contraception works about 50% of the time. "