"... whether in a meeting or, as a New York Post article over the weekend claims, in the middle of a make-out session in the Map Room with the late daughter of former Vice President Walter Mondale. The Secret Service presidential detail would have stopped him.... 'The inner perimeter is 100 percent controlled by the presidential protective division,' said a former supervisor of the presidential protective division, who asked not to be identified by name. And if Byrne or any uniformed officer had been posted near a room the president entered, he would have been moved at least 15 yards away, to the outer edges of the security bubble — not quite what Byrne describes in his book: 'I stood guard, pistol at my hip, outside the Oval Office, the last barrier before anyone saw Bill Clinton,' according to the Post, which has been teasing excerpts of the book."
From "Secret Service veterans denounce anti-Clinton tell-all book" (in Politico).
ADDED: If what these Secret Service veterans are saying is true — and Byrne as a uniformed officer couldn't possibly been in the position he claims he was in and to have seen what he says he saw — then Bill Clinton should sue. Bill Clinton could show that Byrne knew that what he was saying was false, and that would meet the "actual malice" standard that applies when a public figure sues for libel.
BUT: I'm assuming that if Byrne didn't see what he says he saw, the stories are false. It's possible that the stories are true, but Byrne isn't a first-hand witness. And I can see why Bill Clinton wouldn't want to get embroiled in those questions (and others, like what damage do these additional stories do to his reputation).