John Travolta's true sexuality has been an open secret for decades now. In Time Magazine's 1991 exposť on Scientology, they mentioned that the cult may be blackmailing Travolta to stay with them. Here's the quote:
"Screen star Travolta, 37, has long served as an unofficial Scientology spokesman, even though he told a magazine in 1983 that he was opposed to the church's management. High-level defectors claim that Travolta has long feared that if he defected, details of his sexual life would be made public. "He felt pretty intimidated about this getting out and told me so," recalls William Franks, the church's former chairman of the board. "There were no outright threats made, but it was implicit. If you leave, they immediately start digging up everything." Franks was driven out in 1981 after attempting to reform the church.
"The church's former head of security, Richard Aznaran, recalls Scientology ringleader Miscavige repeatedly joking to staffers about Travolta's allegedly promiscuous homosexual behavior. At this point any threat to expose Travolta seems superfluous: last May a male porn star collected $100,000 from a tabloid for an account of his alleged two-year liaison with the celebrity. Travolta refuses to comment, and in December his lawyer dismissed questions about the subject as "bizarre." Two weeks later, Travolta announced that he was getting married to actress Kelly Preston, a fellow Scientologist."
I believe Carrie Fisher also outed Travolta in 2009 when she told Out magazine that "(straight women) don't really care that John Travolta is gay." Travolta was not amused.