They can't, the Catholic church considers Matrimony a Sacrament and there are certain qualifications to receive every sacrament and 2 persons of the same sex are not qualified to receive the Sacrament of Matrimony.....a female is not qualified to receive the Sacrament of Holy orders, a non-baptized person is not qualified to receive most Sacraments......it just doesn't work.
With all respect...since when do Radical Lefties give a darn about written doctrine? They defy and ignore biblical text; so why are you confident that they will obey Catholic-specific doctrines?