While same-sex marriage and workplace discrimination against gays have been attracting headlines recently, there's another, lower-profile front in the gay rights battle: the fight over second-parent adoption for same-sex couples.

There are seven states that bar a resident from adopting an unmarried partner's biological or adoptive child if he or she is gay; there are are nine that allow it if the same-sex couple is married, has a civil union or domestic partnership; and 12 that allow it regardless of the couple's marital, civil union or domestic partnership status. The remaining states do not have clearly-established laws on the subject.

Here's a fantastic map the Post's graphic artist Cristina Rivero did showing the breakout across the United States:

In June 2012 the American Civil Liberties Union filed a federal constitutional challenge to the ban in North Carolina on behalf of six gay and lesbian couples. Leslie Cooper, a senior staff attorney at the ACLU's Lesbian Gay Bisexual Transgender & AIDS Project, called the expansion of second-parent adoption crucial because "it's important, most of all, for children of same-sex couples to be able to have legal ties to both of their parents."

In a worst-case scenario, Cooper said, a child might not be able to stay with the surviving parent if his or her legally-recognized parent died.

The issue of second-parent adoption first surfaced in the early 90s, when gay couples raised the subject in state courts and there was a series of appellate court decisions. In several instances, the courts ruled that second-parent adoption is allowed under the existing statutes; in two cases, Connecticut and Colorado, the courts ruled it was not allowed but the legislature later passed laws allowing it to happen.

Opponents of gay marriage such as Traditional Values Coalition president Andrea Lafferty said restricting second-parent adoption to heterosexual couples makes sense because children in families where both parents are of the same gender "are still not getting the male or female nurturing relationships they need."

"Most Americans understand that children need a mother or father," Lafferty said. "When you have a gay or lesbian it is still a single-parent household."

Peter Sprigg, a senior fellow for policy studies at the Family Research Council has a similar take, though he emphasized that his group is opposed to non-married heterosexual couples adopting children as well as same-sex couples.

"We believe that having both a mother and father who are committed to one another in marriage is in the best interest of children, including adopted children," Sprigg wrote in an e-mail. "It is not in the best interest of children to be raised either by cohabiting couples (which lack the stability of marriage) or by same-sex couples (which deliberately exclude either a father or a mother from the home)."

Cooper said that while the North Carolina suit is "our first case of this kind" because it's a federal constitutional challenge, "we’re certainly trying to expand second-parent adoption across the country. We’re pursuing all different avenues to doing that, including legislative ones."

In the coming years the United States may get a little more clarity on this question, too. But it won't come without a political fight.