A great urban legand, but when you think about the mechanics of it it is clearly a very unsound claim. Assuming it takes a few seconds to even process a thought, this would leave little scope for men to develop any non-sex related thoughts. (Spare me any pathetic stereotypical jokes about how this is in fact the case)
Some studies suggest most men think about sex at least once each day. I have to admit that it can often be significantly more. But once every 9 seconds. That would mean 6400 sexual thoughts per day (based on 16 waking hours).
Also if you don't believe me a quick google search reveals a multitude of sources dispelling this urban legand, yet no strong evidence supporting it.
Time to put this fanciful claim to rest.