That, or just give up any hope of having a rational discussion with your parents.
Anyone who buys a Mac because of the above reason, because it's an overpriced, overhyped pile of crap made by Apple, or because it's an extra-shiny facebook interface.
Yeah, while it's true that not many mac viruses existed in the past (because they consisted a relatively small percent of the computer device population), in the modern day there are certainly enough out there that you should set up some sort of virus/malware blocker on any mac you own.
Personally I own a mac, mainly for 1 big reason, compatibility. With a little bit of work you can easily get your mac set up to dual/triple boot and then run
every program (which is nice if you want to say, play a game, develop a computer program, and write some music all on the same computer). Windows, on the other hand, has no easily done way of doing this, and any attempts to do say take a
lot more work.
That said I can certainly see the attraction of a mac for older people that just want to use their computers for email/word processing/internet browsing. Macs do a much better job at hiding/limiting access to the interior of the computer, which IMO leads to a bit of a more polished look and less "crazy settings and buttons you shouldn't push" to quote my Grandmother.
For those of us who actually understand how to use those various access points a windows is much better and more affordable (unless you are going for compatibility, as I mentioned above), but I can certainly see the appeal for those who aren't very computer literate.
Just felt like putting in my two cents there.