That is not exactly true. Some people consider "emotional affairs" to be cheating just as well. Not many people would jump for joy if they found love letter emails/chats online written from or to their mate even if the other person is located in another country.
Most people upon witnessing their mate (only) holding hands while out for a walk or making out in the park (with no sex) would consider it cheating.
Cheating is a combination of (secrecy), (deception), and (betrayal).
If some people would consider it a form of cheating if they caught their mate flirting or using sexual innuendo in a conversation. Everyone draws the line where (they) see fit. Each of us is entitled to have our own "deal breakers" or lack there of.
Bottom line is a couple needs to discuss their expectations of one another and not "assume" they are on the same page. One person may think flirting is "no big deal" or spending one on one time socially with exes or people of the opposite sex is perfectly fine. Truth is there really is no "right" or "wrong". There is only "agree" or "disagree". Ultimately everyone is looking for someone who (naturally agrees) with them.