It all has to do with Muhammad's amazing modesty. He was the most modest person to have ever existed. He said so himself.
And he was especially modest when it came to exposing his private parts. No one, not even his favorite wife, Ayesha, ever saw him naked. Good thing, too, because if they did, they'd go blind.
Lady Ayesha … said, "I never saw the private parts of the Messenger of Allah." … The Prophet ... asked Imam Ali to ensure that no one other than himself should wash him (his covered body after his death). He told him, "No one has ever seen me naked, and if it were to happen he would become blind." Ash-Shifa, p.48So please don't look at the cartoon below. If you do it will be the last thing you'll ever see.