Avast can expand this web content filtering from any category of web filtering specially in porn site?
Not really sure what your asking but if you want to block porn sites then use your browsers content filtering or parental control features.
Cause Avast have a phishing site filtering maybe they can expand this into a large web filtering not only a Phishing site but from most sites to be blocked like porn, weapon, nude, etc. According to each category.
I suggest that you try OpenDNS as your DNS server as it has a dashboard feature that you can define what categories you want to block at DNS level, so it never gets to your browser. See http://www.opendns.com/home-solutions/parental-controls/.