Alexandra, online dating can be cruel and impersonal. Many people just disappear, as you said. It's not considered rude. What you decide to do is up to you. If you'd feel better telling someone you're no longer interested, do it. And if they don't like that, just rise above their comments. You are merely being courteous, as you were brought up to be.

Don't take their comments personally.
Don't take their comments personally.
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