Need to be able to search for multiple checkboxes

For example, in the Babes search engine, under "Butt", there are checkboxes for "Natural", "Fake", and "Unknown". Since "Butt" is a very new search field, almost all the results are for "Unknown", but let's assume a lot more data eventually gets entered for "Natural" or "Fake". So, what if I want to search for both "Natural" AND "Unknown", just to exclude the known "Fake" results? Currently, I can't -- as soon as I check one checkbox, the others disappear. This is not how checkboxes are supposed to work -- they're supposed to be multiple-choice, for the example I just gave.
 
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