“HERE’S A fascinating story,” the longtime host of NBC’s Tonight Show With Jay Leno told his audience last week, mentioning a recent Iowa Supreme Court ruling “that a dentist could legally fire his female assistant because he found her too sexy, too attractive, and a threat to his marriage. She didn’t do anything. She’s just good-looking.” The payoff: “Hey, I’ll bet that’s what happened to me here at NBC!” It’s the sort of joke that is rapidly becoming a staple of Leno’s monologue. (On Monday’s show, riffing on the royal baby, he confided that he was “a little bummed out,...