We can agree that there are a vast number of Windows viruses (At least 68,000 according to Symantec - not including worms and spyware) - and Mac OS X is relatively virus-free.
However, since Apple reported yesterday that Mac OS X has over 14 million active users now, your explanation doesn't withstand scrutiny. Viruses have been sucessfully developed for platforms with a much smaller user base than Mac OS. Many hackers have tried to develop effective Mac viruses - and they've all failed.
Most experts attribute the Mac's security record to superior design and engineering.
The reason that Windows has so many viruses is because it is a soft target. It's much easier to infect Windows because it is designed poorly. With a little motivation, any pimple-faced teenager can write a Windows virus and spread it to millions of PCs.
I like that. Let's see, 14 million is 1.8% of the market, which means maybe ~784 million Windows boxes. Now if Windows has 68,000 viruses in the wild then that means the Mac should have at least 1,200 viruses by now. Let's give a BIG fudge factor to our guests due to the smaller marketshare, say by one hundred, so maybe the Mac should have at least 12 viruses out there. Nope, still not.
Sorry dudes. UNIX has been fighting off attacks in a networked environment for over 30 years, and the developers have learned how to secure the OS far better than Windows, which barely learned how to network about 12 years ago.