The Catholic Archdiocese of Boston, in response to criticism it was sanctioning a celebration of Gay Pride month, announced yesterday that it had ordered a South End church to cancel a Mass scheduled for next weekend that was themed, “All are Welcome.’’The decision outraged members of the lesbian and gay community.“I think that’s horrible, just horrible, that they would cancel,’’ said Marianne Duddy-Burke, executive director of DignityUSA, an organization that advocates for participation of gays in the Catholic Church.