Why would a man constantly lie about being single when he is not?

What’s the point of lying to another woman that he is “single” while also being romantic/sexual with her knowing he has a girlfriend at home. Why do some people do this instead of just leaving their girlfriend if it has come to such a point? But when caught he’s all “guilty”and “remorseful” at least at that moment…😂 I never understood that
Why would a man constantly lie about being single when he is not?
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