The event was to celebrate and discuss the book written by Hwang Sunwoo and Kim Hana, both 47, about their life together as single women in South Korea. But a man in the audience was there to offer criticism. He told the two women that they were making the country’s birthrate, already the world’s lowest, even worse. Their book, he argued, would encourage other women to follow suit.

       “The irony was that the man, of our age, was unmarried himself,” Ms. Hwang said. “More people choose not to marry or not to have children, but it’s usually women to blame.”

       South Korean society is deeply patriarchal and built around the traditional idea of family. Many government benefits — tax, housing, insurance and other incentives — are tailored for families. In return, families are expected to shoulder much of social welfare, such as caring for sick or elderly relatives.

       Such longstanding norms may yet change. On Thursday, South Korea’s Supreme Court ruled that same-sex couples qualify for the national health insurance’s dependent coverage, a decision that rights activists hoped could pave the way for legalizing same-sex marriage in the country.

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       But with millions of South Koreans shunning the institution of marriage, the family-centered support system is rapidly unraveling. The nation’s quality of support network — measured by whether people have someone to rely on in a time of need — is the lowest among developed nations. South Korea also has the highest suicide rate among those nations.

       For Ms. Kim and Ms. Hwang, who say they are not romantically involved with each other or anyone else, the solution is to redefine the concept of family. Their 2019 book, “Two Women Live Together,” has become a best seller, and their subsequent weekly podcast, “Two Women Talk Together,” attracts hundreds of thousands of listeners — giving voice to South Koreans, especially women, who have challenged the traditional family structure by forming cohabiting unions outside of marriage.

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