Why is my boyfriend borderline obsessed with the woman he cheated on me with?

Six months ago, my boyfriend cheated on me with a friend of a friend. I forgave him and we have been making an effort to repair our relationship (or so I thought). I recently found out that for these same past six months, he has been emailing her (she has him blocked everywhere after she found out that he was in a relationship) and keeping tabs on her via burner social media accounts, LinkedIn, and through their mutual friends.

He told me that it was just about sex with her and that he meant nothing to him. Why is her so obsessed with her?
Why is my boyfriend borderline obsessed with the woman he cheated on me with?
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