While I agree that many crop circles are man made, they are most certainly not ALL man made. You can find tons of resources and information that can illuminate this subject for you....my personal favorite being Freddy Silva's book,
Secrets in the Fields.
Most of the arguments against crop circles being "paranormal" come from the following sources.
1.
The Doug and Dave story.
2.
The obvious hoaxes. (which usually do not even compare with the real phenomenon)
3.
The ability for some experts (Colin Andrews included), to be fooled by hoaxes.
The main aspect of crop circles which lends credibility to the assertion that they are not all hoaxes is actually their history. Crop circles, as well as similar depressions in snow, rock, and other mediums have been reported all over the world for many many years. It is no doubt that the great majority of the circles are hoaxes nowadays, in fact they may be almost entirely hoaxes nowadays, but as is the case with cave paintings depicting gods descending from the heavens....if even one of them is real, or depicting a real event...then we have some serious deliberation to do as a species.