I believe Cliff is a straight man, who chooses not to share his life with a woman - and there are plenty of guys like that, but hardly any as famous as he is, and therefore worthy of endless pondering.
I shard a lunch table with Cliff once on a TV shoot, and we got into conversation about faith and relationships. Cliff commented that he is used to the constant questioning of his lifestyle and sexuality, and it doesn't bother him at all. He said that as a committed Christian he has far more trouble making time to read his bible every day than to live without a relationship, and I believe he was being completely honest.
Fair enough, he is not likely to reveal any intimate secrets to a journalist, but i still believe that the reason why there have been no skeletons in Cliff's closet (pun intended) is because they don't exist - a boring conclusion, but then the truth often is.