There's nothing wrong with having sex with a woman while she's on her period. The only issue is that it's gross, but as long as you don't mind cleaning up the mess, and the girl is okay with it, then go right ahead. Something to keep in mind, if you have sex during menstruation, pregnancy will not occur. When a woman menstruates, her uterine wall sheds, thus it will not take a fertilized egg. Be aware though, she can get pregnant if it's just spotting and not actual menstruation, and she can get pregnant if it's towards the end of her menstrual cycle.