Garance Franke-Ruta blogs for The Atlantic:

"...In surveys conducted in 2002 and 2011, pollsters at Gallup found that members of the American public massively overestimated how many people are gay or lesbian. In 2002, a quarter of those surveyed guessed upwards of a quarter of Americans were gay or lesbian (or "homosexual," the third option given). By 2011, that misperception had only grown, with more than a third of those surveyed now guessing that more than 25 percent of Americans are gay or lesbian. Women and young adults were most likely to provide high estimates, approximating that 30 percent of the population is gay. Overall, "U.S. adults, on average, estimate that 25 percent of Americans are gay or lesbian," Gallup found. Only 4 percent of all those surveyed in 2011 and about 8 percent of those surveyed in 2002 correctly guessed that fewer than 5 percent of Americans identify as gay or lesbian.

... The Williams Institute at UCLA School of Law, a gay and lesbian think tank, released a study in April 2011 estimating based on its research that just 1.7 percent of Americans between 18 and 44 identify as gay or lesbian, while another 1.8 percent -- predominantly women -- identify as bisexual. Far from underestimating the ranks of gay people because of homophobia, these figures included a substantial number of people who remained deeply closeted, such as a quarter of the bisexuals. ACenters for Disease Control and Prevention survey of women between 22 and 44 that questioned more than 13,500 respondents between 2006 and 2008 found very similar numbers: Only 1 percent of the women identified themselves as gay, while 4 percent identified as bisexual."

She concludes:

"One thing's for sure: it's hard to imagine the fact that so many think the country is more than a quarter gay or lesbian has no impact on our public policy."