Martin Luther King Jr. did cheat on his wife, according to Encyclopedia Britannica, which cites FBI monitoring files made public in the 1970s through the Freedom of Information Act. Since then, a series of rigorous scholarly investigations into his life have verified his participation in what Bio.com describes to as “adulterous relationships.”
As described in the Pulitzer Prize-winning King biographer David J. Garrow’s piece “The FBI and Martin Luther King” published in The Atlantic on July 1, 2002, U.S. Attorney General Robert F. Kennedy first gave the FBI permission to wiretap King’s phones on October 10, 1963.
The reason for this was the rumour that Stanley David Levison, one of King’s most influential advisors, was a significant American Communist Party member. After meeting King in 1956, Levison started giving him informal advice.
King continued to consult with Levison despite warnings from those close to him to distance himself from him. Kennedy’s mistrust of King was sparked by his friendship with Levison, and the FBI’s wiretapping of King to ascertain his alleged communist party membership—which was never established—led to the agency’s discovery of his extramarital affairs.
Although there is ongoing debate on Martin Luther King’s legacy, the Encyclopedia Britannica notes that it is generally agreed that, despite his flaws and limitations, he was a visionary leader who was fiercely committed to the cause of civil rights attained through non-violent methods.