Not saying this is the reason why its gone but..

Porn on a forum increases the weighted "bad word" value and causes a lot of web filters to block it. Not to mention most porn that is mostly stolen/pirated/cross-linked. Plus i'd imagine there are sponsors that don't want their ads popping up above adult material.

All in all i support the decision, plus its not like there's a shortage of porn sites on teh interwebz.