Sex when your friend's in town? Sure, say Susan Kim and Elissa Stein, the authors of Flow: The Cultural Story of Menstruation. We talked to them about their lively, irreverent book, and about why something so natural still gets such a bad rap.

MC: Why do we tend to be mum about our monthlies?

SK: They've been viewed with fear since the days when people didn't know where the blood was coming from.

ES: And religion didn't help. Ancient faiths found it unclean. Even the Bible warns men against shaking hands with a woman during her period, because she's impure.

MC: And having sex while menstruating was certainly out of the question--which still feels true today.

SK: For some guys, the idea of seeing blood on themselves gets at a very deep Freudian fear of castration.

MC: It makes women anxious, too.

SK: Right. Because we're sexually vain: I don't want to slink across the floor in a negligee with a streak down my leg. Even though it might be good for you--orgasms during your period can help prevent endometriosis.

ES: They can also relieve cramps and release oxytocin, the love hormone, bringing feelings of calm and well-being at a time when you could really use them. And some women report increased libido during their periods. Why would you deny yourself?

MC: But what about the mess?

SK: Buy a cheap Fieldcrest towel in dark red, throw it down, and give it a whirl. You might like it. You might not. But it's not like you're sacrificing a baby to a Druid god.

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