Most relationships begin with some type of flirting or sexual innuendo.
While flirting may not be "cheating" (I doubt any of us would be happy if we caught our significant others flirting with someone.) Relationships have ended over it. No one wants to see their husband/wife flirting.
I've never heard of anyone asking their spouse/mate if they would have problem with them flirting with others,
People flirt for different reasons. Sometimes it's to stroke their own ego in order to prove to themself that if they were back on the open market "they still got it going on!" They want to know they are "attractive to others" in addtion to their mate. Some people do it to get help or gain some type of advantage. Others do it to "test the waters" to see if there might be a mutual attraction.
The best definition I've heard for cheating is "If you are doing or saying something you wouldn't do or say with your mate sitting next to you then it's cheating or at the very least inappropriate.
Cheating usually involves keeping "secrets", lying directly or by omission.
Each of us has our own "deal breakers" or draw our own lines as to what is or isn't acceptable in a "monogamous relationship". It's probably best for both people in a relationship to lay out their expectations.