Gallup gives a much lower number of 16%. Probably half of those are lying too

Just looking into the 40% poll quicky, it seems like it was designed to produce a high number. From the article

Barna produced the 124-page study in conjunction with the Cultural Research Center at Arizona Christian University and Foundations of Freedom, a non-profit entity that promotes traditional American values. The results differed significantly from a February Gallup poll that showed just 5.6 percent of U.S. adults of all ages are not "heterosexual or straight," though that poll said that 7.6 percent of respondents refused to answer the question and another 5 percent said they had "no opinion" as to whether they are heterosexual or straight. The Gallup survey concluded that one in six Americans ages 18-23 were gay, lesbian, bisexual or transgender.