Girls, Is it cheating if I flirt with a single woman, if I already have a girlfriend (she hasn't asked and I haven't said)?

It's not in person, only via messaging - hasn't gotten racy or sexy, and nothing's happened between us - we've complimented each other and like each other, but neither party has done anything about it. I wouldn't leave my girlfriend for her, or cheat on her - but should I have said something sooner?

Girls, Is it cheating if I flirt with a single woman, if I already have a girlfriend (she hasn't asked and I haven't said)?
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