How do you tell a girl that you aren't attracted to her anymore?

For context let's say a guy gets in a relationship with a girl and she gains weight. And he doesn't want to just break up with her because her loves her. How does he go about talking to her about it with her without upsetting her to much?
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By the way this happened to me in high school. It's not a current issue I'm dealing with.
How do you tell a girl that you aren't attracted to her anymore?
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