The down sides... its messy. There are ways around that tho and some people are actually in to that. There is an odor but if the woman is clean it shouldnt be sickenly so. Contrary to popular belief, blood is not lubricating it is actually sticky and dry so extra lube might be needed increasing the mess but there again, shower. The cervix is open just enough to pass the uterine sluff so a condom is a good idea as the chance of introducing infection is higher. All in all tho, there really is no real reason to avoid sex during her period. Orgasm helps her with cramping and the hormones involved with increased sexual activity might help fight those ones involved with pms.