There is no operating system that is 100% secure, but Windows tends to make exploiting it far too easy.
Exploiting weaknesses in modern UNIX systems, like Mac OS X, is a task for hacker elites. Teenagers with basic scripting skills need not apply.
So, generally - Macs are more difficult to exploit through a network. And it isn't a difficulty born of obscurity. It is a result of the UNIX underpinnings that severely limit programs based on their permissions. This of course means the primary means by which Macs are vulnerable to malicious code is when the user is duped into running the software, and even then - the operating system will ask for admin authentication if the program in question tried to do anything out of the ordinary.
Im sure its on purpose.... *rolling eyes* at mac fanatics.
Nobody is perfect and its asinine to believe the exploits are intentional.