The one holy Roman Catholic Church teaches the following:
1. Homosexual acts (sexual acts between persons of the same gender) are intrinsically evil and always gravely immoral.
2. The homosexual orientation (“an exclusive or predominant sexual attraction toward persons of the same sex,” CCC 2357) is a grave and inherent moral disorder, since the orientation directs the person toward acts of grave depravity. This orientation is contrary to the plan of God for humanity. God never wills a person to be sexually attracted to members of the same gender.
3. Neither a true natural marriage, nor a true Sacrament of Marriage, can exist between persons of the same gender. Therefore, so-called ‘gay marriage’ (same-sex marriage) is not in any sense a marriage before the eyes of God or His Church.
4. Laws establishing a legal form for gay marriage, or for its near-eqivalent in any type of civil union, are unjust laws that contradict the eternal moral law of God.
The Catechism of the Catholic Church, n. 2357:
Basing itself on Sacred Scripture [Cf. Gen 191-29; Rom 124-27; 1 Cor 6:10; 1 Tim 1:10], which presents homosexual acts as acts of grave depravity, tradition has always declared that “homosexual acts are intrinsically disordered.” [CDF, Persona Humana, n. 8] They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.
Whoever knowingly chooses to reject the definitive teaching of the ordinary and universal Magisterium against homosexuality, commits the sin of heresy and is automatically excommunicated from the Church.