The organisation had appealed to the tribunal for permission to exclude same-sex couples, arguing that it would otherwise have to close its adoption service because its supporters would stop donating money.
Catholic Care, a Leeds-based charity that facilitates about five adoptions a year, argued that restricting its service to heterosexual couples was in line with section 193 of the Equality Act 2010. The section allows discrimination on the grounds of sexuality if this is "a proportionate means of achieving a legitimate aim".
However, Alison McKenna, principal judge of the charity tribunal, sitting with two lay members, ruled that the discrimination would cause same-sex couples to suffer a "significant detriment".
The tribunal found that there must be "particularly weighty" reasons to justify discrimination on the basis of sexual orientation and said the charity had not demonstrated that its donors would stop supporting it if it allowed same-sex couples to use its adoption service.
The tribunal also said Catholic Care had not yet explored all the alternatives to closure, and that expert evidence contradicted the charity's argument that if it were to close, fewer children would be adopted.
The Charity Commission ruled in November 2008 that the charity could not change its objects to exclude gay couples, a decision upheld by the charity tribunal in June 2009. The High Court judge Mr Justice Briggs ruled in March 2010 that the commission must reconsider its verdict, but in August the commission reaffirmed its original decision. The charity's latest appeal was against the commission's August ruling.
Benjamin James, a solicitor at the law firm Bircham Dyson Bell, acting on behalf of Catholic Care, told Third Sector it would be possible for the charity to appeal to the Upper Tribunal, but it had not yet decided whether it would do so.
He said the Upper Tribunal could ask the Charity Commission to reconsider its decision and give it a direction that would make clear the grounds on which it could do so.
The charity has 28 days in which to notify the tribunal if it wishes to appeal against the decision.