I guess im annoyed because I can think of 3-4 times in the last 6 months where im chatting with a girl and they are being rly flirty with me, so of course if im interested, i reciprocate. Then just before anything happens, i get "oh, by the way, I have a boyfriend.."
Either they are not flirting (flirting is relative), or they want to have an affair with you. Talking and laughing with you is not flirting. A regular look across the room is not flirting, but a sexy "come hither" look might be. It's all subject to interpretation.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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