No one has said "Mac OS X cannot get viruses." It COULD, if a virus existed. There simply ARE no viruses in the wild at this time that can affect Leopard and Snow Leopard. That could change tomorrow, but the world would quickly find out about it.
That's why antivirus apps can't protect you from a Mac virus. They can't protect you from something that doesn't yet exist, because they don't know what to look for. In theory, a code could be written to exploit some vulnerability in Mac OS X. What that might look like, what the vulnerability is, and what it might do as a result is completely unknown at this time.
No, you can't get malware simply by visiting a porn site. If you give your email address, you can get spam. If you create a user name and password for the site, that has nothing to do with your administrator password on your Mac (unless you're foolish enough to use the same password!)
Currently, the only threats are trojans. For a trojan to work, you must first download it, then install it, entering your administrator password. You will also get a popup warning when you first launch a downloaded app, asking if you're sure you want to run it. As you can see, there are many safeguards built into Mac OS X. If you get a trojan, it's because the user did something foolish.