mac viruses are much rarer then windows (but do exist), however I have seen an increasing number of mac trojans that could still infect viruses.
However a trojan requires user interaction and needs to be installed, often people get trojans when some popup showing their computer is infected, a video doesn't work and requires you to install a "codec" to view and gives the address on where to get it, there are a number of more examples.
Now if any of those porns sites ask you to install something on your mac to view it, close it right away as it could be a trojan that infects your mac.
Also if by some reason you think you have any type of maleware on your mac, I would suggest an antivirus scan. Some free ones that exist for the mac are clamxav (http://www.clamxav.com/
) or iAntivirus (http://www.iantivirus.com/