Most famously, [Guy Fawkes Day] also bequeathed us "guy". At first this meant the effigy of Guy Fawkes traditionally burnt on the bonfire (children once constructed guys and begged money with them for fireworks with the cry "a penny for the guy!"). But it's also where "guy" in the sense of a person comes from - it was originally applied to a man of grotesque appearance, like a bonfire effigy, but when it was taken to the US in the late nineteenth century it turned into a neutral term for a man, more recently a person of either sex.
(Emphasis mine.) Nice to know the origins, right, guys?
I believe that Quinion is right, but the use of guys in a sex-neutral way is limited to the plural, as in you guys -- a vernacular second-person plural. My sense is that guy as a singular still refers to a male person. Yes?