Why do women sleep with their best friend’s man?

Just heard another woman’s horror story about this. Before that I saw a girl's video on YouTube about how she was dating a guy from work and her girlfriend tried to tell her he was no good, then at some point the girlfriend went in for the kill and slept with him.

Why is this a thing?

Why do women sleep with their best friend’s man?
Why do women sleep with their best friend’s man?
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