This is approximately the percentage of the world population he had sex with, too.
But Actually ...
Nobody has any clue. That 10 percent figure might actually be as low as 2 percent, or as high as 20 percent. Polls keep being conducted, and the only thing they've determined so far is we have no fucking idea.
The problem is obvious -- we're relying on surveys in an era when it's still not OK to be gay in huge swaths of America, let alone the rest of the world (for example, if we went purely by self-reporting, there has been only one gay player in the 94 years the NFL has existed). So in those surveys, gays could hide their sexual orientation, refuse to answer the question, or simply not know the answer themselves yet -- it's not like it's a binary question (as Kinsey himself could have told you).
In his own unique way.
Being homosexual isn't like being Asian, or tall, or having brown hair -- hiding it is as easy as not saying anything. Try as he might, even Daniel Day-Lewis can't convince a census taker he's, say, a pretty Japanese schoolgirl. But he could convince them he's a homosexual -- even a happily married man could turn out to just be in the closet. So even if the pollsters and scientists spent the next year lurking in the shadows and closely observing every human's dating and sex habits, they still wouldn't get their answer.