Is it possible a man is lying to his girlfriend about loving her if he still teases and flirts and stares at another woman in their friend circle?

A woman who liked him and whenever he sees her, even if the girlfriend is there, he gives her attention? I mean if he really loved his girlfriend surely he would not be giving any mixed signals to another woman? I mean maybe if the woman was a total stranger I could understand a bit of light hearted flirting, but this woman is not a stranger.
Is it possible a man is lying to his girlfriend about loving her if he still teases and flirts and stares at another woman in their friend circle?
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