Good example. I personally didn't love the show but I know MANY friends male and female who were obsessed with it. I don't know that I would go so high as to say 80%, but I wouldn't be surprised if it was near there. As for the reason for it's popularity, while buffy wasn't overly sexualized, she was still an attractive female lead in a series made by men dealing with themes that interested men. The themes they dealt with were very rarely focused on gender at all (which is refreshing by the way), and to be fair, the series was made by Joss Whedon, and anything that guy touches is amazing.
If that's a common truth and not an outlier though, why did similar shows like 'charmed' and 'angel' (despite a male lead and being a spinoff of buffy) not garner a majority male viewership?