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Katy Perry didn't dump John Mayer because he cheated, but rather because she was afraid he would. So she did it preemptively. From In Touch Weekly:
"She always thinks John is either going to cheat or is cheating," the source explains.

And her fears might not have been so far-fetched. Now, sources confirm exclusively to In Touch that John got cozy with 24-year-old spin instructor Lauren McHale last spring during a brief break with Katy ... For Katy, her uncertainty was enough to call things off, says the first source.  

"She'd rather have kept dating, but he wanted to marry her. He pressured her. He gave her a ring and made the relationship more serious, which scared her because of his past."
If everyone were to act preemptively we wouldn't be able to get anything done, but well, good on you, Katy. Now you get to be single and date other people who might just do that to you.