Google Ad Planner show some site visitors demographic data, like the visitors gender.
Take a peek at Stackoverflow's Google Ad Planner.
As a Web Developer I'm concerned where we are leaking this data to Google. This has anything to do with our use of Google Analytics or it is unrelated?
There is anything we can do to prevent Google to obtain such data?
The more specific demographic data won't come from actual users visiting the site, but will come from mixing together all of the various subject matter variables with market research data to come up with a set of approximations.
I have found this site with an answer:
http://www.google.com/support/adplanner/bin/answer.py?hl=en&answer=98132
Google Ad Planner combines information from a variety of sources, such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms; it doesn't contain personally-identifiable information.
In addition, Google Ad Planner only shows results for sites that receive a significant amount of traffic, and enforces minimum thresholds for inclusion in the tool. For more information about how Google protects privacy, please refer to our privacy policy.