Definitely not - as you get older you'll notice that quite a lot of people kiss each other on the cheeks to say "Hi" (here in Europe it's especially popular, particularly in France, Italy and Spain!), it means absolutely nothing but a friendly "Hello"
In my mind anything that does not involve feelings and doesn't go beyond a certain limit (for me it would be a "proper" kiss, not necessarily with tongues but more than just a peck on the lips) can't be considered "cheating" - the general definition is that of actually sleeping with someone but obviously when feelings are involved (i.e. someone falls for another person) then IMHO it doesn't matter whether they actually go as far as to sleep with the person because they've obviously got feelings for someone else.
Each of us has a different definition of cheating, and what counts is that you respect the feelings of your gf/bf unless they are totally ridiculous and over the top (say "You can't even talk to a guy if I'm not there").
That said I am sure most people will at one or more points in their lives have a drunk (or even sober - you don't necessarily have to be tipsy to make a silly mistake) but meaningless kiss/snog with someone.
If you feel you've "crossed the line" then it's a good idea to tell your gf/bf about it - more likely than not they won't consider a kiss on the cheek as a breach of their trust