What is a polite way to reject someone under the following scenareos, so that they won't feel hurt?

1. After immediately meeting them and finding that what you can see of their face/body is not attractive to you.
2. After talking to them, and not finding enough mental compatibility.
3. After seeing them naked, and liking their mind but not their body.

What is a polite way to reject someone under the following scenareos, so that they won't feel hurt?
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