Writer Robert Randolph has written a book, You'll Never Spa In This Town Again, in which he reckons he saw the Grease star, 56, getting it on with men in Los Angeles steam rooms.
Randolph blasted Travolta as a serial cheater who makes philandering golfer Tiger Woods, 34, look like "a boy scout", reports the Daily Star.
He even calls the movie legend's marriage a "total fraud" and blasts him as a "cheating dog".
But Travolta has denied the allegations. He has fired off a legal letter to celebrity gossip website Gawker after it published Randolph's accusations.
Yesterday, the star's lawyers demanded the site withdraw the "false and outrageous lies" which have spread worldwide across the Internet.
Attorney Marty Singer said the sleazy accusations were "blatant defamatory lies".