I don't know if the articles claims are true, but I can tell you your ideas about why they are wrong are not correct. First of all, just because McDonald's outnumber porn stores in your town, does not mean that that is true of the whole country. Second of all, just because you see more McDonald's around, doesn't mean that some unseen stores don't outnumber them. You could live all your life in a town and not see such a store, unless you looked for it, or stumbled on it.
Regarding the McDonalds vs. Bookstores claim, the article itself states there are 15,000 adult bookstores. For their greater than 3-1 claim to be true there would need to be fewer than 5,000 McDonalds. Thats only 100 McDonalds per state. Outside of several states such as Alaska, Rhode Island, Wyoming, and maybe 2 others I suspect that it is safe to bet that all the others have well in excess of 100 McDonalds. I would venture to say that it is likely that the 5,000 number can be reached east of the Mississippi!