At 30, the writer Emily Witt found herself single and heartbroken, but also suddenly intent on examining the mythology around how life for women is supposed to be. “Demographics have changed, people get married later or they don’t get married at all,” she said recently, drinking rosé at a bar in Brooklyn.

In considering questions like why she was not married or almost married (and why many of her friends who wanted to be married were also not married), Ms. Witt, who has written for the London Review of Books and The New Yorker, and is a contributing editor to T: The New York Times Style Magazine, recalled thinking that “technology had changed. Social mores had changed to accept a wider range of sexual practices. And it felt like the protagonist in some ways, the main person experiencing all of this, was women.”

Thus began her quest to understand the consequences of these changes. The result is her book, “Future Sex,” to be published Oct. 11.

Along the way, when she would talk about what she was working on, “certain editors — male editors — have commented on my ‘memoir,’” said Ms. Witt, now 35. “An editor said to me, ‘It seems like every woman has to write about this at some point.’ Um, yeah, because it’s one of the most important things about being alive right now?”