ďAn August 2002 Gallup Poll asked the public to estimate what percent of American men were gay, and the percent of American women who are lesbians. While the answers ranged widely, the average estimates by the public were that 21.4% of all men are gay, and that 22% of all women are lesbians. About a quarter of the public thinks that more than 25% of Americans are gay or lesbian. 

Analysis shows that younger Americans give higher estimates of the number of men and women that are homosexual than do older Americans. Republicans and conservatives estimate lower percentages than do Democrats and liberals."

The Gallup Organization - In-Depth Analysis: Homosexuality - What Percent of Americans are Homosexuals?