There are public block lists that are intended to block porn sites, although none of them will be 100% effective.
You can also use regular expressions in Pi-hole to block domains containing keywords normally associated with porn sites.
https://docs.pi-hole.net/ftldns/regex/tutorial/
There are also filtered DNS services that provide "family friendly" filtering. Using one of these filtered services as the upstream DNS server for Pi-hole may meet your needs. OpenDNS is one.