I own both a PC and a Mac...The real question is where is the OS X version? Bioshock released a Mac version shortly after launch, now nothing? At least that is a legit reason for not wanting to buy it.
What's the point when you can save yourself money by buying a proper gaming computer? Not only that but a PC runs games perfectly and is widely supported, unlike Macs where you'll be lucky to receive AAA titles and even then typically years after release. No, you can't seriously claim the lack of a Mac version is a legitimate reason not to purchase a game when you own both platforms.