I remember hearing this rumor back in school from people who thought they knew. It usually came up when people were drawing pot leaves on shit, and someone would offer up that female plants had 5 leaves, and males had 7 or more.
Of course, now that I've spent more time researching, and have now started my own crop, I see that the number of fans on a leaf seem to have more to do with maturity of the plant. First ones have 1 fan, then 3, then 5, then 7 as the plant gets bigger.