Oohh... Look at all the informed opinions...

Anyways, I think that it is not "cheating" per se, to do something like fantasize about another person than one's mate. Lucid Dreaming is just another method of fantasy, some would say, just a very vivid one. Whatever you believe about LDs, what matters is that all of this is in the mind. Therefore there is nothing physically harmful about it.

Here's the issue: The definition of "cheating" may change from person to person, as does the very definition of "wrong". What you consider cheating is all your opinion, and all anyone else can tell you is what the average opinion of society is. On a subject such as this, I doubt you're going to get a clear answer...