I think it's a concern if you have it in your mind to flirt with someone when you're in a relationship. I have never so much as batted my eyelids at other men when I'm in a relationship, let alone actual flirting. My feeling is that if you're cool with your partner flirting, and your partner is cool with you doing it - then you guys do you. However, it is cheating to me. If you're about to do something that you would hide from your partner - think twice.