An interesting hypothesis that Symantec makes, in relation to the mention made by joe53, that "religious and ideological sites were found to have triple the average number of threats per infected site than adult/pornographic sites" is:
"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."
...and I'd have to say that makes sense to me. Probably right on the mark. I'd also like to point out the distinction made which can easily be overlooked. That is, their statement here:
...triple the average number of threats per infected site than adult/pornographic sites...
The way I understand that as it reads, is that for each infected web site there were found to be triple the average number of threats, and not triple the risk or rate of infection simply by visiting a religious or ideological web site as compared to a typical porn web site. Note also that no attempt is made to define "infection", which could quite possibly include something as simple as adware.
Let's reduce their statement to it's base and say...there is just one religious/ideological web site out there. Just one. And that one is infected. Now...the way the statement reads, you could rightly assume that the infected site has three times the number of infections on that particular web site than any particular porn site that may be infected.
Also of note, is that to compare such a thing is something on the order of apples to oranges. If you consider that the three "infections" on some religious/ideological web site could be adware and compare that to a rootkit infection on a typical porn site, then you can make a better decision as to which is the greater risk.
In my mind, it seems they have constructed the headline to imply that there are three times as many infected "religious or ideological" web sites than there are porn sites which also seems to imply that one has three times the risk of infection visiting ANY of those religious or ideological web sites over any particular porn site...and nothing I can think of is further from the truth in that regard. The typical Internet user who glances over headline may very well read this that way.
In the decade or so that I've worked to remove malicious code from a users system, I will say without reservation that I've found an overwhelming majority of them came from porn sites...peer to peer comes in at a very close second place.