Your formula would bar some works of Chaucer.
Not everyone who asks to have the filter removed is searching for porn. For example, a patron researching breast cancer would probably be blocked. Also, we have had patrons blocked from employment websites because the words "under 18" were found.
It isn't as simple as you seem to think it could be.
It isn't as simple
If the parent is there with the kid, it gets pretty simple. :>
Are you saying Chaucer can't be read to a City Council?
Library computer are not to be used for porn.
Figure out another use for them.