I highly recommend everyone read “Considerations Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Persons” published July 31, 2003, by the Sacred Congregation for the Doctrine of the Faith, signed by Joseph Cardinal Ratzinger, Prefect.
A few excerpts:
Marriage is not just any relationship between human beings. It was established by the Creator with its own nature, essential properties and purpose.
Marriage is holy, while homosexual acts go against the natural moral law. Homosexual acts close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity.
Laws in favor of homosexual unions are contrary to right reason and promote homosexuality, thereby weakening marriage. Legal recognition would result in changes to the entire organization of society, contrary to the common good; would modify the younger generation’s perception and evaluation of forms of behavior; would obscure certain basic moral values and cause a devaluation of the institution of marriage; and would redefine marriage, making it an institution devoid of essential reference to factors linked to heterosexuality — procreation and the raising of children.
The approval or legalization of evil is something far different from the toleration of evil.
Even where homosexual unions have been legalized, clear and emphatic opposition is a duty.