Today the Talmud explores the doctrine of “taah bdavar Mitzvah”. If a man is making a mistake but is preoccupied with trying to fulfill a Divine commandment we sometimes go easy on him. On Shabbat, sacrificing the Passover offering is allowed (and required), but slaughtering other animals is Biblically forbidden. If a man slaughters a different animal under the mistaken assumption he is slaughtering his Paschal lamb, then Rabbi Yehoshua lets him off the hook for performing a forbidden labor on Shabbat based on “taah bdavar Mitzvah”.
The Talmud shares an amazing example:
Rabbi Yoḥanan said: If one mistakenly engaged in sexual intercourse with his wife while she was menstruating, he is liable to bring a sin-offering. But if he engaged in sexual intercourse with his sister-in-law who was waiting to become his wife through levirate marriage while she was menstruating, he is exempt, because the act of intercourse itself is a mitzvah (Divine command).
A man whose brother died without children is obliged by Torah law to marry his deceased brother’s widow (Deuteronomy 25:5), see Judah and Tamar and Ruth. This levirate (Latin for brotherly) marriage could be consummated through intercourse.
The Gemara explores why this man thought he was doing a mitzvah, and concludes that the wife was already pregnant, so there was not command of be fruitful. Our sages explain that the joy of a husband and wife being together is a mitzvah, based on Exodus 21:10 “If he marries another wife, he must not withhold from the first one her food, her clothing, or her conjugal rights”. Even if he has fulfilled the minimum level, a man is expected to notice when his wife is interesting and give her more. So intimate relations is always fulfilling the Divine will, except for the time period when she is about to menstruate. Then there is a requirement to abstain since she may see blood. The Bible forbids sexual relations for a week beginning with a woman’s period.
But why is a man guilty of sleeping with his wife but not his widowed sister in law? The Gemara explains that with regard to his sister-in-law, he is still shy in front of her and uncomfortable asking her whether she is close to her expected menstruation, whereas with regard to his wife, he is not shy in front of her, and so he should have asked her.
Yes, a husband is expected to know or ask when his wife is going to menstruate, and also ovulate. Humans have a concealed estrus, meaning unlike some animals female ovulation is not obvious. However, the Talmud today shows us that in a healthy marriage the husband is well aware of his wife’s cycle. He needs to know not only to avoid sexual contact during her period, but also to treat her appropriately when she is ovulating and then while she is preparing for her next period. He sees how she behaves and dresses and understands where she is in her period, and if there is a chance she is about to menstruate she is expected to tell him. A wise man learns to anticipate his wife’s behaviors and needs and uses this information to treat his wife well.
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