Viagra nation

I watched the newest vidcast of diggnation over the weekend, as I usually do. It is a reasonably good show about the most popular stories covered on digg.com. Alex Albrecht is too cool, mostly because he drinks beer, plays pencil and paper D&D, is on the totallyradshow, and kicks Kevin Roses arse at Call of Duty 4. That is some serious geek street cred.
This past eppisode though, they revealed some ignorance about the mechanism of Viagra. Kevin Rose (seriously check out how badly he is pwned by Albrecht on CoD4) thought that you could take Viagra and have a voluntary errection where you would have to 'be in the mood'. Someone from the audience who identified themselves as a 'life scientist' said that it was not voluntary. Well, Kevin Rose was right and the 'life scientist' was wrong. Strange since I find Kevin Rose usually talks out his ass.
As I understand it viagra mearly inhibits the degradation of cGMP, which directly initiates and maintains an errection. However it is produced in response to nitric oxide. The nitric oxide is produced upon sexual stimulation, therefore even with viagra it should still be a voluntary thing. That's got to be a relief for those who take the drug since you are supposed to take it 4 hours before you think you will need it. That would be a long 4 hours if it was involuntary.