1. While you are menstruating your body has good hormonal balance and therefore, you are protected from getting osteoporosis, breast or uterine cancer.
2. During menstruation, as Miffiy said, your body has a chance to give the vagina a good clean.
3. Although I don't recommend unprotected sex (getting carried away is stupid, not romantic) it is a good indicator that you are not pregnant.
4. While your are menstruating your are also having discharge (yes that also stops when you reach menopause) which means you are less prone to infection and have less difficult getting your body to lube up during sexual arousal.
5. While you are menstruating (assuming everything is in good order ovary wise) you are probably in a good place for having children - one of those strange urges that women seem to get as time goes by.
6. Menstruation provides a good excuse for not engaging in sexual intercourse.
7. Menstruation also teaches you very quickly about the importance of personal hygeine!