I suggest you read Cecil Adams (From the Straightdope) had to say on the matter:

http://www.straightdope.com/columns/030502.html

I think I'll quote the end, after he reveals everything about King:

"As every reasonable observer has commented, neither King's sexual wanderings nor his scholarly misdeeds detract from his core achievement. By continually publicizing black grievances while putting a palatable, nonviolent face on resistance to jim crow, King paved the way for the landmark civil rights legislation of the 1960s and a major turnaround in public attitudes about race. But there's no getting around the fact that he was a complex and deeply flawed man. Was he a great American? No argument here. Was he a fraud and a hypocrite? He was that, too."

Cecil says it all there. He wasn't perfect, he was a man. He had his flaws, but his accomplishment is still great, nevertheless those flaws.