Of course they're not. Ten or so years ago, for example, there were some WDEF viruses that affected Macs, as well as MS Office macro viruses that were cross-platform. And obviously if you run Windows on an Intel Mac under Bootcamp or any of the virtual environments such as Parallels, it's just as vulnerable as if you ran it on a PC.
There still has not been a virus in the wild that would affect OS X, but there will be some day.
True, but not as well as a PC can. Mac users need to be careful about sending files to PC users, because they might be infected. That's the main reason to run anti-virus software on a Mac. However, the PC virus won't run on a Mac, and therefore can't spread itself from the Mac. A large number of worms and viruses spread by infecting Windows email hosts and sending themselves to people on the address list. Random addresses on the list are spoofed as the "sender," so if the spoofee is a Mac user, someone might erroneously think they got the virus from a Mac.
Most troubling is your wishing viruses and worms on other people of any sort.
Well, maybe, but since any script kiddie can make a Windows virus, what bragging rights go with doing that? Wouldn't it be more impressive to write the first virus that will in fact (and not just in theory in the lab) infect Macs running OS X? And now that about 14% of laptops sold in the US are Macs, the argument from obscurity seems outdated, if it ever was valid.
Windows used to by default leave practically everything open to malware, all sorts of ports open, and give root level of permissions to .exe files and the like. Vista, for whatever good or bad things one might say about it otherwise, has supposedly tightened up a lot of the security holes.