Why did Donald Trump inexplicably defend the size of his penis in Thursday's debate? Because he's unnaturally sensitive about it? Because, as Jeet Heer suggests, it's part of a venerable history of monarchs and presidents? Because Hillary Clinton would be the first penis-free president, so it's a good way of contrasting himself?
Yes to all of the above, I suppose. Plus the fact that Trump is a self-centered boor. But this is all background noise. The real reason, which Trump understands instinctively, is simpler.
Trump's supporters love him not so much for his policies but for his promise of toughness. Without that, he's nothing. And to his supporters, toughness is deeply tied up with virility and manliness. This includes all the affairs, the succession of young wives, the supermodels, and the fact that he brags endlessly about it. Most of his supporters don't precisely approve of all this stuff, but they nonetheless admire it when it comes from someone so successful. If that's what it takes to save the country, then that's what it takes.
So Trump made it clear that his manliness is quite intact, thank you very much. This is, if you'll pardon the pun, all part of the package. It's true that Marco Rubio fired the first shot a few days earlier, but that never came up in the debate. Trump brought it up out of the blue. He wanted to bring it up.
Everyone in the press mocks him for this nationally televised display of crudeness, but Trump brought it up because he wanted to assure his supporters he was a tough guy. And I'll bet it worked.