This sounds about right:
The inaugural results of a new Gallup question -- posed to more than 120,000 U.S. adults thus far -- shows that 3.4% say "yes" when asked if they identify as lesbian, gay, bisexual, or transgender.
[. . .]
Gallup chose the broad measure of personal identification as LGBT because this grouping of four statuses is commonly used in current American discourse, and as a result has important cultural and political significance.
For decades the national discussion of issues of sexuality has been contaminated with the bogus claim, mostly pulled out of thin air by the Kinsey Institute (or perhaps Kinsely has been misquoted), that 10 percent of the population is gay. Relying on the truthfulness of people who self-identify is risky, but the 3.4 percent figure is consistant with what other studies have indicated. I realize some people are invested in minimizing the probable number of LBGT and that others have reason to make it seem as big as possible, but there are tremendous public policy issues involved, so we all should be interested in getting the number as right as possible.