What are the long-term effects of staying with someone who cheated on you, even if you think you can forgive them?

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One of the things that I noticed when I found out my ex was cheating on me was that I didn't want to sleep with her any more, and that really pissed her off, as if she didn't quite understand why I might feel that way.
What are the long-term effects of staying with someone who cheated on you, even if you think you can forgive them?
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