This is from the initial post on this thread, which if you read the whole article included bisexuals. The 6 million figure is found in the study submitted to the Supreme Court by gay groups.
The unusually candid statement about the relatively low number of homosexuals in the population appeared on page 16 of the brief. The text contains the assertion, "There are approximately six million openly gay men and women in the United States, and 450,000 gay men and lesbians in Texas." After the national figure there appears a footnote, number 42 in the brief. The actual footnote at the bottom of the page reads as follows (in its entirety):
The most widely accepted study of sexual practices in the United States is the National Health and Social Life Survey (NHSLS). The NHSLS found that 2.8% of the male, and 1.4% of the female, population identify themselves as gay, lesbian, or bisexual. See Laumann, et al., The Social Organization of Sex: Sexual Practices in the United States (1994). This amounts to nearly 4 million openly gay men and 2 million women who identify as lesbian.
I can't speak for Australia but in the U.S. youth as young as 12 are self-identify as gay, bi, lesbian, which is why some high schools have gay youth organizations.