Lying when already caught?

So I just recently found out this girl who is supposed to be "Sis" is sending my boyfriend pics.. I asked him because that girls friend let me know what she was doing. I confronted him about it and he lied and kept lying. Then when I told him I would give him one last chance go come clean he did. But why lie if you already caught? And now he did that do I just let it go because he came clean or do should I worry about being cheated on and lied to?
Lying when already caught?
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