Most Helpful Guy
If you respond to someone's else's flirting is basically you saying "I'm not completely fine with my bf/gf, I'll give this a shot and see how it goes". It's a 5 sec impulse, you don't think about it, but in the end it means you are wanting to leave some doors open "just in case". That's cheating, yeah.
Most Helpful Girl
Flirting is emotional cheating. You're crossing a line from friendship and showing romantic interest. The only person you should be interested in flirting with is your SO. If not then it's probably best not to have one.