The avast site blocking isn't really designed to block categories of sites, it has to be a specific website using the full URL. Whilst wildcards * can be used in the URL you still have to have the major content of that (I have inserted spaces to prevent the example link being active) w w w.*porn*.com is such an example.
That won't block all p o r n o g r a p h y sites only those that might have porn in the domain name. This is not an efficient means of blocking, there are many better tools to do that. This is better done at DNS level, using something like OpenDNS, which has blocking by category.
There also hosts file managers that are geared up to block such sites.
