As a gay person, I can tell you that debates of these statistics - "How many people are actually gay?" - is a popular discussion in gay circles. Many gay people think that the number is closer to 20% because, frankly, they've experienced things that lead them to believe that, such as having sexual encounters with self-identifying straight people, and confessions of people that they wouldn't tell anyone else.
But what is gay? Just one same-sex encounter out of ten thousand opposite-sex encounters? What if you like it sometimes, or at one point in your life, but not others?
In this, I defer to the Nazis as always. They were very very meticulous about their sociographic statistics, what with their racial theories and all. They found that around 8% of the German population was gay. That number seems about right to me.