Is it ethical to tell your opponent how you are going to checkmate them? I play online chess at 3 days per move. I like to tell my opponents how I'm going to checkmate them if they will never win. That way, that I won't waste time continuing the game. I also like to be told if I will be mated for the same reason.
The majority of the users find that insulting. Is it really insulting to tell your opponents how you are going to checkmate them so they can resign fast? I don't play against titled players, so they don't see the checkmate coming unless you tell them. Meanwhile, titled players fight to the end online often, with many games ending in checkmate, unlike real games where you very often resign.
It is not a question of ethics, but more about being courteous.  Chess is a game where it is impossible to separate the joy of the game from the competitiveness/ego aspect so by declaring a forced win in N moves, you are effectively asking your opponent to resign immediately even though he hasn't seen the forced win yet owing to his lesser faculties/skill set. 

Try doing that kind of thing at work/real life .. you'll be out of a job in no time (or get punched in the face) :)

Your opponent may be weaker than you and he may not be able to calculate like you do, but give him the chance OR let him play the game out till the bitter end.  You can't just force somebody into resigning by brow-beating them, for God's sake :)

Beyond the probability (however small) that you may be wrong / You may have missed an in-between move or desperado yourself, you are essentially coming off as a jerk. The rare exception is that you are coaching somebody (as part of a mutually agreed session, not one where you suddenly decided to act like a coach! ) where you can afford to point things out to them and not come off as arrogant.