What counts as cheating?

Hmm....so I have a thought..and a question. God no, I'm not in a relationship or anything, but it just crossed my mind...what counts as cheating in a relationship? Obviously, sleeping with someone else would be there on everyone's list, but would kissing with no emotional crap be considered as cheating? What about no physical contact but just thinking about another person while in a relationship with someone else? I have come across so many people saying "he cheated on me by kissing another woman" or "she cheated on me when she thought about another man". Seriously though, where do people draw the line, and why?

Does that mean that if I'm in a relationship, I cannot even look at another man? Because that would just be barbaric...like some nations in the Middle East. What if I look at another man and go "whoa"? Is that cheating? Is that wrong?

Good for me, I don't do monogamy!