One reason people tend to overestimate the percentage of Americans who are gay and lesbian: we clump up in cities. We're a relatively tiny minority, it's true, but even if you accept the lower estimates (2%) there are still more gay and lesbian Americans than there are Jewish Americans (1.7%). And while we're evenly and randomly distributed throughout the population and the country at birth, almost all gays and lesbians migrate to bigger cities as young adults. Even if small towns and rural areas were tolerant and accepting places—and they're generally not—there just aren't going to be enough queer people in a small town to create a viable dating scene. Young gay men who aren't willing to settle for furtive blowjobs from closet cases in truckstop restrooms quickly move to the closest big city—Chicago, Atlanta, Seattle, New Orleans, San Francisco, New York, Miami, etc.
So while we may represent "just" 2% of the total population, we represent much a higher percentage of the population in any given large American city, a fact that often leads people to overestimate our total numbers.