Why do Christians think cheating on your girlfriend is a sin or a girl cheating on her boyfriend is a sin?

I've talked to many Christians that believe this nonsense. Cheating on your girlfriend is NOT a sin neither a girl cheating on her boyfriend. There's zero evidence of this from scriptures.
It is NOT adultery which is breaking the sanctity of marriage. Where you're cheating on someone you're NOT sinning because it is impossible to say dating someone is adultery
Why do Christians think cheating on your girlfriend is a sin or a girl cheating on her boyfriend is a sin?
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