Well, being that I have been a MAJOR example of being cheated on...here's my .02
If you are in an exclusive relationship, and any part of your body(the good ones!) touches other than your significant other, it's cheating, from kissing, all the way up(or down shall we say?) to the whole she-bang...hehehe
If you are just dating (and able to see other people, which should be agreed on upon right after you get together) then go for it, kiss away, cuz he's prolly doing the same thing.
MY personal preference, regardless if I am just dating or in an exclusive relationship, my lips and body are for that one and only....otherwise, it's just plain confusing. You should be with someone who makes you happy, if not, you shouldn't be with them anymore.