The Socialist majority in Parliament will try to override President Anibal Cavaco Silva's veto.
The couple had cared for the baby for a month before finding out she is not their child.
Adoption was a workaround that protected many LGBT couples, but now it complicates the quest for wedded bliss.
As the nation's high court considers the case, it recalled the Alabama Supreme Court's decision, which voided the woman's adoption of her ex-partner's children.
Phyllis Gilmore, head of the state's child welfare department, denies there's any bias at work, although she says families with a mother and father are best.
Right now there are nearly 400,000 children in the U.S. without a family, and more than 100,000 of them are eligible for adoption.
Judge Tim Fox's ruling inititially applies only to the three couples who filed suit, but he's indicated he's likely to expand it statewide.