The question comes toward the end of the Boris show. The man who will be the next prime minister of the United Kingdom has mussed his golden hair, insulted a Scottish fondness for deep-fried chocolate bars, and deployed trademark wit in describing the importance of tackling crime as “basic hygiene for Conservatives.” It is his last chance to persuade members of the Conservative Party to back him as leader before voting starts the next day. And the audience in Perth, Scotland, is loving every moment. Then, after 41 minutes, a tall woman in a black dress makes her way to the microphone. “My name is Flora,” she says. “I am undecided, but leaning Boris, but one thing gives me pause and it’s this question: Does a good prime minister need to be a good husband and father?” For five seconds the only sound is that of several hundred well-heeled Conservatives emitting irritated groans, followed by a smattering of boos. No one is surprised. Least of all the candidate, who responds: “I’ve been asked all sorts of questions over the last 20 or 30 years, and I just don’t comment on that stuff, if that’s all right, because what people in this country want to hear is what my plans are to get Brexit over the line —” Applause overtakes the rest of his answer. This audience has heard enough of allegations of affairs, illegitimate children,...