Whether it's cheating or not depends on the couple involved and their views. I think you should ask your partner before doing something like that and if they are ok with it then that's fine, but if not then you should respect their wishes.
I recently discovered that whilst my ex was with me he played spin the bottle and kissed a fair few girls numerous times. At the mature old age of 18 :P
He was drunk(which we all know isn;t a real excuse) but i was interested to find out how many people considered this cheating and how many people considered this harmless fun!
oh, and i mean that if you're not present at the time (and don't get told until your single :P)
it is absolutely cheating. unless you were there and told him its alright, it is cheating. any sort of romantic/sexual contact with someone outside of the relationship without consent from your s/o is cheating. be glad he's your ex lol.