I'm not sure if that is fair. There are SO few gays in society, generally, that it would be nigh on impossible to ascertain that number.
As for unbiased studies, that would be the best, but studies are RIDDLED with flaws. The LEAST reliable is the survey study. That is probably what would be attempted.
But, in the end, it really is about money, isn't it? It usually is.
I can assure you that finding a statistically valid sample of male homosexuals in San Francisco would not be a problem.