CHICAGO (AFP) - A US federal judge struck down a gay marriage ban in the mid-western state of Michigan and found there was no "credible" evidence that heterosexual couples make better parents on Friday.
The decision is the latest in a number of court rulings to find gay marriage bans unconstitutional.
The challenge to Michigan's prohibition was brought forward by two foster mothers who argued that their children were harmed by the fact that the marriage ban prevented them from being jointly adopted.
The issue of whether gay and lesbian couples make good parents was thus put on trial.
US District Judge Bernard Friedman dismissed as "unbelievable and not worthy of serious consideration" the testimony experts called by the state in an attempt to prove that children raised by same-sex couples "fare worse" than those raised by heterosexual couples.
He noted in a 31-page opinion that the state did not challenge expert testimony that "every major professional organisation in the United States whose focus is the health and well-being of children and families" has concluded that children raised by gay and lesbian parents "are not disadvantaged". Aside from being a violation of constitutional guarantees of equal protection under the law, Friedman also ruled that the gay marriage ban "fosters the potential for childhood destabilisation". Without the ability to be adopted, a child could lose both parents should their legal guardian die or become incapacitated.
Michigan's Republican Attorney-General Bill Schuette immediately filed an appeal.
Federal judges in Kentucky, Ohio, Oklahoma, Texas, Utah and Virginia have recently ruled in favour of marriage for lesbian and gay couples, as has the New Mexico Supreme Court.
The rulings follow a landmark Supreme Court decision in June finding that couples in same-sex marriages were entitled to the same benefits and protections as their heterosexual counterparts.
Marriage laws are governed by individual US states, nearly 30 of which have amended their constitutions to ban same-sex marriage.
Efforts to give same-sex couples the same rights as heterosexuals have gathered steam in recent years in the United States, where most Americans now back gay marriage.