I've never had a virus from a free porn site. I use a free virus scanner called Avast, which has saved my neck on a number of occasions - and has never needed to save me from free porn viruses!
With the completely overpriced nature of UK pay-to-view websites - and the smug attitude of some posters here - it wouldn't surprise me if the nonsense about free porn sites being riddled with viruses was being spread on purpose by those with a business interest in the UK online porn industry.
I'm sure there are nasty obscure free sites out there which intend to infect your computer, but there are also a number of reputable free sites which have been around for years, which don't. They pay for themselves via the advertising revenue, so the last thing they'd want to do is frighten off their own visitors and kill the site's reputation by spreading viruses!
Also, porno movie sites based on the Youtube model are becoming more common, so these days there's no need for you to even download a file onto your computer.
I would suggest that rather than using scare tactics to lure customers into the overpriced UK online pay-to-view porn market, UK webmasters shift their business to US or foreign servers and charge a more reasonable price to the English customer.