Well AFF is unique in that it's explicitly a hookup site, and that might naturally skew men. I'm not sure an even gender balance would even help anything. With 1,000 men and 1,000 women, you'd still have the most attractive women flooded with messages.
Among the 26,939 users with a UK email address, for example, there are just 1,596 who identified as female: a ratio of one woman to every 16 men.[1]
[1] http://www.channel4.com/news/adult-friendfinder-dating-hack-...