You mention that you are willing to accept a 5% error rate. 5% is a bit higher than the Error Rate from Trolling of most surveys (see http://slatestarcodex.com/

You mention that you are willing to accept a 5% error rate. 5% is a bit higher than the Error Rate from Trolling of most surveys (see http://slatestarcodex.com/2013/04/12/noisy-poll-results-and-reptilian-muslim-climatologists-from-mars/ http://slatestarcodex.com/2013/04/12/noisy-poll-results-and-...). Do your users have an incentive to falsify their gender which would push it higher than that? Since gender is relevant, I assume you're making either a dating app, or an app that feeds into some sort of beureacracy. If the former, then people have an incentive to enter their real gender if they want to get any use out of the app. Linking to Facebook is a bad idea since people are usually kinda wary of linking their dating profile to other social media. If the latter, then facebook is not going to satisfy your stakeholder. You should consider asking for some form of ID or credit card.