Turbo, you're framing this issue in hypothetical terms. I'm framing it based on what we know from real-life experiences with these products.
The Mac is secure. It is secure because it is built that way, and is superior to any personal computer system on the market. It doesn't need extra software to make it secure. This isn't a hypothesis. It is a fact.
The Windows-based PC is insecure. Thousands of viruses have infected millions of real computers and have done real damage. Many of these computers had third party software on them. It is insecure because Windows is a piece of crap, built to become obselete the day you purchase it, and built to satisfy the non-thinking consumer and the corporate executive who only thinks about short-term costs, not long-term consequences of purchases.
Further, on a Mac, the software and hardware work hand in glove. One protects the other.
Single- or multiple-sourcing isn't an issue when your system is vulnerable whatever method you choose. Multiple-sourcing is not, in and of itself, a superior method of protecting a computer when it is still flawed in the real world.
"Real-life experiences" do not in fact cover everything that potentially could happen. As an analogy, did you know that a commercial passenger jet has never attempted a water landing under full control? But because we frame critical issues in "hypothetical" terms, we still make them carry life rafts, floating seat cushions, etc. Would you want those devices taken off your next intercontinental flight just because they deal with "hypotheticals"?
Security means planning not only for those contingencies that have occurred, but also those that haven't. With regards to a Mac, that means that security requires being prepared for a security failure in a single piece of software (in this case, OSX) even though such a failure may never have occurred before, and even though the software may be designed to avoid a failure. Unless you want to claim OSX is perfect, making it unique in millions of years of human endeavors, it's irresponsible to ignore any such risk.
It is insecure because Windows is a piece of crap, built to become obselete the day you purchase it, and built to satisfy the non-thinking consumer and the corporate executive who only thinks about short-term costs, not long-term consequences of purchases.
On a side note, you really don't do yourself any favors by insulting people with whom you're having a discussion. If such is more your speed, I guess I could just save my time discussing security and just call Mac users a bunch of "gay commies".