Having sex during a period will not harm a woman. The menstrual fluid consists of blood and tissue from the lining uterus, with some bacteria that are normally present in the vagina. The menstrual fluid, therefore, doesn't contain anything dirty or harmful ,so it wont irritate the penis or cause any other problems. The time during which a woman menstruates is the least fertile period in the woman's cycle, so it's one of the best times to avoid pregnancy.