According to Encyclopedia Britannica, FBI surveillance files released in the 1970s under the Freedom of Information Act showed that Martin Luther King Jr. did cheat on his wife. A number of serious academic studies of his life conducted since then have confirmed his involvement in what Bio.com refers to as "adulterous relationships."
As chronicled in an article in The Atlantic on July 1, 2002, called "The FBI and Martin Luther King" written by the Pulitzer Prize-winning King biographer David J. Garrow, U.S. Attorney General Robert F. Kennedy first authorized the FBI to wiretap King's phones on Oct. 10, 1963. The motivation was the suspicion that one of King's top advisors, Stanley David Levison, was an important member of the American Communist Party. Levison met King in 1956 and began informally advising him. Though King was warned by those close to him to disassociate himself from Levison, King continued to seek his advice. It was King's association with Levison that led to Kennedy's suspicion of King, and it was the FBI's wiretaps on King to determine his affiliation with the communist party, which was never proven, that led to the FBI's discovery of his extramarital affairs.
The Encyclopedia Britannica points out that though contention continues over Martin Luther King's legacy, it is generally acknowledged that although he was flawed and fallible, he was also a visionary leader who was deeply committed to the cause of civil rights obtained through non-violent means.