This article says it's only like 2%. I find that incredibly hard to believe. I will be fair though, I live in the Seattle area, which might be higher than the national average, or at least higher in self-reporting. But way more than 2% of all people are I know are gay, and that's an unbiased sample since I'm mostly closeted. It's more like 8-10%.
The more fascinating part of that article is how bad people are at percentages. I don't think people are overestimating the number of gays, they just don't know how much 25% is.