A federal district judge struck down a ban on same-sex marriages in Pennsylvania on Tuesday, giving American gay rights campaigners their third judiciary victory in just a week.
Gay rights activists welcomed the decision on a lawsuit filed by a widow, 11 couples and two of their teenage children.
On Monday, a federal judge in Oregon made a similar ruling as did a judge last Wednesday in Idaho.
Eighteen states and the District of Columbia currently allow same-sex couples to marry, and courts across the country, including in Oklahoma, Utah and Arkansas, have struck down state laws banning marriage for same-sex couples, ACLU said.
A landmark Supreme Court decision last June found 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, more than 30 of which still ban same-sex weddings.