Viagra and Blindness

OK, I'm sure we've all heard by now that Viagra has been associated with blindness in some men, and we've all heard the jokes. But what I want to know is why nobody's noticed that there is one Viagra-related activity that men could engage in that is reputed to make one blind. Maybe it's all those commercials showing a man and a woman together that have made us ignore the possibility that some of these men were, ahem, flying solo?

I'm just sayin'...