What's the difference between being unkind about someone and giving your opinion?

What do you think is the difference between being unkind/rude and giving an opinion on something or someone? Where do you draw the line?

For instance, my friend says she doesn't think Christiano Ronaldo is attractive. Is that rude or giving her opinion? What if someone were to say that someone you both know is not attractive? Is that then rude or giving your opinion?

What's the difference between being unkind about someone and giving your opinion?
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