I mean, any reason for not wanting a relationship is a good reason because a relationship where one person doesn't even really want it is going to be a shit relationship.

That said, I might still judge you if your reasons are shitty.

Why would I care about someone else not dating someone. That's insane behaviour

You have the right to refuse for any reason, that doesn't absolve you of how you will be perceived though.

my favorite is people who say they don't care about the reason, and then use the same reason to reject people.

or who whine about being rejected for a reason, and use that same reason to reject people.