Infidelity like beauty is in the eye of the beholder.
Early on I believe it's important for two people to discuss what they consider cheating. If you are not on the same page then there is likely to be problems.
"Secret friendships", flirting, sex via webcam, using sexual innuendo, online relationships...etc might be considered cheating by someone who is in love. As Dr. Phil once said, "If you wouldn't do it or say it with your mate sitting next to you then it's probably cheating." Certain things should be saved for your mate only.
Then there are other couples who have "open relationships" and they are ok with their significant other or spouse having sex outside of their primary relationship. As I stated from the outset you really have to know your mate and set the "ground rules" or expectations. It's a mistake to "assume" everyone feels the same way as you do.