Because she knows where your mouth has been.

I never have, nor ever will, use someone else’s toothbrush.

Tossing her salad is gross but it gives her pleasure. Using her toothbrush is gross, but gives her no pleasure. There's the logic, shit-eater.

Logically it shouldn't make a difference. But it does. And you have to learn to respect that. Otherwise, you end up serving chocolate mousse shaped like dog turds and wondering why no one wants to eat them.

that needs a conversation of, you made a mistake, you're sorry, it won't happen again, but withholding sex as punishment is not appropriate and will not be tolerated in a relationship. nip that shit now

This could be solved by buying new toothbrushes. If this solution doesn’t satisfy her and she still insists on a sex ban, break up immediately.

It doesn't. but people are quirky. Just apologize, tell her you don't understand but you respect it, and that you'll never do it again.