Symantec's 'Internet Security Threat Report', published earlier this year, makes the following observation (p. 33):
"...religious and ideological sites were found to have triple the average number of threats per infected site than adult/pornographic sites. We hypothesize that this is because pornographic website owners already make money from the internet and, as a result, have a vested interest in keeping their sites malware-free – it’s not good for repeat business."
Internet Security Threat Report (PDF, Symantec)