Disclaimer: I Do Not Own Harry Potter.
Their marriage had gone through a lot, and others would have crumbled under the strain. After all these years she wasn't going to throw the towel in just because he got tempted by some young bimbo.
She would forgive him because she had no choice otherwise everything else they went through was for nothing. Everything and everyone she lost to Voldemort would mean it was all for nothing.
Narcissa would always forgive him because if she didn't all those of years her life were worthless she were to walk away now.
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