Stuff like this, while done with good intentions, can threaten to entrench discrimination by having different sets of rules and expectations for people based on their gender. A better solution would be to give 12 extra holiday days per year to everyone, but make it known that it's acceptable to take them under those circumstances.
Men don't get a day of pain and cramps each month so I don't see why we should get the day off. Periods aren't fun, and I don't think men deserve a day at the golf course in exchange.
It applies to maternity/paternity leave too. Until maternity and paternity leave are required to be equal by law, there will be a bias against hiring women, due to the simple mathematics. If the 'risk' is equal for a man or a woman, the bias is eliminated.