I agree wholeheartedly with the commenter above...no matter how many great things a person does in his or her lifetime, none of them excuse cheating. It may not be any big deal to the rest of the world, but I can assure you it is a very big deal to the spouse being cheated on, and their kids if they know about it. Unless both spouses are cheating, in which case they deserve each other.
Apparently MLK strayed multiple times; it seems it was a way of life for him. It is much easier to forgive sin in others when it is unexpected - i.e., when temptation strikes and they fail to resist it. When sin is a way of life, it is obvious it is planned and there is no repentance. There can be no forgiveness without repentance.
The most condemning factor in MLK's case is the fact that he was a minister, supposedly representing the Word of God. I would stake my life on it, that his sin was an extremely big deal to God.
However, the bottom line to all this is it's none of our business, particularly now that he isn't even here to repent. What he did in his lifetime is between him and his God. However, if it were up to me to choose who to honor on national holidays, it wouldn't be someone with such public sins. Appreciate the work he did, but don't honor the man. He doesn't deserve it.