An April report by the Williams Institute revealed that an estimated 4 percent of Americans are lesbian, bisexual, gay or transgender.
Their analysis of 2000 Census data suggested that more than 250,000 children in the United States are being raised by lesbian and gay parents.
The census only asks whether a couple (same-sex or different-sex) have children, counting only the number of "householders with children" and not the actual size of the family.
"It has been consistent since we have been measuring this," said Gates. "If you want to guess the number of kids raised, look at the couples and double it. If they have kids, on average it's two."
Children under 18 include those who have been conceived by one partner or adopted, as well as children from prior relationships and "related" siblings or cousins.
Gates noted that the number of same-sex couples who are adopting has doubled, from 8 percent to 19 percent, even in states where they cannot legally marry, according to research by The New York Times.
Still, more than 80 percent of the children being raised by gay couples are not adopted, according to Gates. And the largest number of children in same-sex families are a result of previous heterosexual marriages.
The Alabama data reflects a growing trend: a large number of children being raised by gay families in the socially conservative southern states, according to the Williams Institute.
Gays and lesbians tend to come out later in life in communities where they are stigmatized, according to Gates. By then, partners may already have children from earlier heterosexual marriages.
Census numbers are only a snapshot of gay families and many more same-sex couples may exist uncounted by the government, say advocates.
Some gay couples may not want to identify as such because of concerns about risking confidentiality. They may call themselves roommates or unrelated adults.
Nonetheless, advocates say census data is important.
"I think this is critical new information and really goes back to beginning of the gay rights movement and Harvey Milk saying, 'Come out, come out, wherever you are,'" said advocate Gaffney, "It's the single most important thing you can do. Let the world know we are here."