Whenever the question is asked and answered as to whether or not flirting is cheating it is usually done so from the perspective of the answerer doing the flirting. Generally speaking cheating contains two components; secrecy and deception.
Is holding hands cheating? Most people would say it is not until they were presented with a photo of their spouse holding hands with someone else walking along the beach.
Someone once defined cheating as saying or doing anything you would not say or do if your mate were sitting right next to you. If one feels the need to hide their actions with others from their mate it's clearly a sign that they do not believe it's the "right" thing to be doing. The definition of cheating is breaking the rules.
The Unasked Question
Rarely if ever does anyone ask their mate if it is okay with them to flirt with others. Very few people would give the "green light" for their spouse, mate, or significant other to flirt with others. Personally speaking I’ve never been in a relationship where my mate told me it was okay for me to flirt with other women. There’s an automatic silent assumption that flirting would not be condoned. This explains why no one ever asks their mate if it's okay.
Most people would consider it disrespectful if their mate flirted with someone in front of them! This implies that we as a society do not believe people in "exclusive relationships" should be flirting. In fact even on a "first date" it would be considered rude to flirt with another person. If you witnessed your mate flirting with someone odds are you wouldn't be happy about it even if you did not call it “cheating”. However you would likely have doubts about how serious your relationship is from their point of view.
Flirting like kissing is something everyone does at the start of all new dating relationships. Odds are you flirted with your current partner before you became a couple. It's a way to "test the waters" to see if there is a mutual attraction. If the other person takes offense at being flirted with one can always pull back by saying: "I can't believe you thought I was serious!" This disarms the offended person causing them to question herself or himself about the flirter's intent and possibly their own lack of a sense of humor. The flirter then moves on to find someone else to flirt with.
The goal is not to have one way flirting. People are looking to engage in back and forth flirting with others. Flirting and sexual innuendo can lead to forming a new secret friendship.
Innocence & Justification
People in a relationships or marriages oftentimes remind themselves of how innocent their actions are and some say; "Just because I'm married doesn't mean I can't have friends." The line keeps moving.
Flirting is not cheating. Having "secret friends" is not cheating, meeting for lunch or dinner is not cheating, holding hands while walking together is not cheating, and foreplay is not sex....etc
Motives & Intentions
Some people flirt simply to prove to themselves they still "have it", the ability to attract others. It's an ego exercise. They need to know if their existing relationship ended they would have no problem attracting a new mate. They want to feel desired by others outside of their mate. However the person being flirted with has no way of knowing it's all in fun.
Intention is invisible both to one's mate and the person being flirted with. Most people will admit they only flirt with those they find attractive or need their help in some way. We also tend to only flirt back with those we find attractive. Very few people are open to flirting with any and every one. The fact that flirting is a selective exercise geared towards specific people we're attracted to or under particular circumstances implies it's not entirely "innocent". Many of us do not question our own motives nor do we take responsibility for our actions in our efforts to have fun. We also do not seek our mate’s permission to flirt. Ultimately only your mate can state if your flirting is "cheating" or a "deal breaker".
“While we are free to choose our actions, we are not free to choose the consequences of our actions” – Stephen R. Covey