According to Taylor Branch's exhaustive, three-volume history of the Civil Rights Movement, Martin Luther King basically used extramarital sex as a form of masturbation. Inevitably, the FBI taps picked up some of these affairs, where according to Branch's book, they got him on tape yelling "I'M FUCKING FOR GOD!!"

There was some judge in DC that put all those tapes and other things under seal in the 70s for 50 years or something like that, so the first we'll get to hear of them will probably be sometime in the late 2020s or early 2030s. [source]