The Church “refuses to consider the person as a ‘heterosexual’ or a ‘homosexual’ and insists that every person has a fundamental identity: the creature of God and, by grace, his child and heir to eternal life.” Each person also has a “sexual identity”: either male or female, man or woman. The Church does not use the term “sexual identity” as some people do, who claim that people have “sexual identities” as homosexuals, heterosexuals, bisexuals, and so forth. Instead, the Church teaches that each male should accept his sexual identity as a man, and each female her sexual identity as a woman; and that means accepting that one is different from and complementary to – and equal in dignity with — persons of the opposite sex (gender).
Aquinas organized his account of the morality of sexual relations around the good of marriage. The good of marriage is one of the basic human goods to which human choice and action are directed by the first principles of practical reason. Sex acts are immoral when they are “against the good of marriage” and therefore unreasonable (and, inasumuch unreasonable, unnatural).