The only time flirting is not cheating is when you have not defined your relationship with your partner, that is why I advise people who think they are in a relationship just because he is there and sends gifts and show love to sit their partner down, have the talk and define what it is, because outsides that everyone is free to do what they want.
This is it o. Nobody in a serious relationship can tell me there's something like harmless flirting. They're only deceiving themselves. A defined relationship that's exclusive has no room for any rubbish flirting.
Flirting is sexual so why have that with just anyone when in a supposed committed relationship