One young woman I spoke to from the Pupa sect of Hasidism — who asked that I not use her name to protect her privacy, as did most of the Orthodox women I spoke with — told me she remembered hearing versions of this story repeatedly from the age of 8 or 9, and recalled going with her eighth-grade classmates to a fair at another yeshiva for girls in her Brooklyn neighborhood. The fair took place in an auditorium that featured a life-size diorama of a mother bathing her daughter eternally in boiling water — a punishment for some undisclosed failure of physical modesty.

For Marcus’s Haredi patients, the entirety of their sexual education has most likely come just before their weddings in classes or tutorials with a kallah instructor, often a rabbi’s wife, who teaches the sacred rules of sexuality. The semiquarantine of menstruation is a crucial lesson, and is usually followed by a summary of what is and isn’t permissible in the bedroom during the remainder of the month, along with some minimal practical guidance, like suggesting the use of a lubricant in preparation for sex.

One morning at the burnished round table where she talks with her patients, Marcus handed me a bride’s manual given out by kallah teachers. This particular book was written for the modern Orthodox; it is relatively progressive. The clitoris, for instance, is mentioned twice. Even so, the overwhelming emphasis is on the wife’s responsibility to keep the relationship on the right side of the law. The Talmud “indicates that during marital relations, the husband may not look at or kiss the wife’s makom ervah,” her private place, the manual warns. The lights should be off, a sheet should cover the couple, the position should be missionary — the wife is charged with keeping sex spiritual, keeping it chaste.

If she doesn’t, a parable in the introduction implies, God’s Chosen may “fall over the edge” of a cliff. “And that book,” Marcus reminded me, “is modern.” Her Haredi women seem to feel that their bedrooms are all but laced with Talmudic “trip wires,” she said, where one wrong move can cause destruction.

When she starts therapy with a new patient, Marcus told me earlier this month, “I feel like I’m in a canyon and need to find some footing.” Searching delicately, painstakingly with each of them, she helps the women to remember moments, recent or running back to girlhood, “when they felt something fluttery, something crinkly in the stomach, funny feelings, a warmth in their genitals,” though they may never have identified the sensation as sexual at all. Despite their flat, resistant voices and bewilderment, she coaxes them to recollect these fleeting experiences, to link them with eros and to understand that the feelings are positive. “That gives me the footing,” she said. “Then we can start pulling ourselves up.” And all the while, she added, “I have to get past their fear: How do I know that where you’re leading me won’t make me want to do terrible things?” She reminds them that God wants a husband and wife to be close and assures them that she is “leading them to a better marriage and to being closer to God.”

Marcus traces her route to her vocation back to her father. He “would tell you that he is absolutely not a feminist,” she said, yet he was appalled that as all his children went through modern-Orthodox schools, only her brothers began studying the Talmud when they reached fifth grade. Girls weren’t supposed to read the hallowed text at any age, a restriction that has since given way in some segments of Orthodoxy. At home, he assigned Talmudic pages to his daughters from the time they were 10; early on, he endorsed paths of independence.