I looked through my search logs and decided to put in a custom-made redirect for "porn" or "xxx". This decision was, to put it politely, motivated by data. Decisively motivated by data. [N.B.: This has since been on my site when I migrated to a search solution that doesn't provide that flexibility.]
My site has so far as I can tell zero SEO to advertise porn, unless you count sporadic uses of the word "porn", which should appear waaaaaayy down the search results list compared to real porn sites, but...
I would tentatively suggest that handling of searches for porn be treated like professional 404 / 500 / ... pages on sites run by people who care about people trapped by porn, and people assaulted by people trapped by porn. In the abstract, coding for every search for porn and only searches for porn is probably as hard as solving the artificial intelligence problem, but in the concrete it's easy. Someone searching for "xxx" is not really searching for a letter signed with kisses! You'll catch much more than 90% of attempts to search for porn simply by filtering for "porn" or "xxx", and less than 1% of people genuinely searching your site (who could still possibly be accommodated by this "missing error page.")
So if you're running a website, do your best to have an appropriate error page for people searching it for porn.
Feel free to forward this on to other webmasters who care about possibly reaching a few of the people searching for porn on their sites. Those visitors are in a deep trap.