You are right - gathering information on a population that goes to great lengths to be invisible is hard and in France as an example you cannot ask someone his sexual orientation in a survey (idem for religion or disease). But we have two new opportunities: one is the Internet and the other one is the introduction of a “third gender” in national censuses in South Asia (Nepal, India and Pakistan). To begin imagining the potential of the Internet, there are 825 million people on Facebook, 12.5% of the world population, and 35% of them disclose their sexual orientation. In addition, small-scale studies have taken place in the HIV context and were deemed representative. Of course this is not ideal but we desperately need data.