According to a recent ABC News/Washington Post poll, the vast majority of Catholics agree with the Supreme Court action which allowed gay marriage to go forward in several states. A whopping 62% of Catholics approved; this number was only exceeded by Non-Evangelical Protestants who approved by 65%; by contrast, just 24% of Evangelical white Protestants approved.
The Church has been repeatedly clear on this point; most notably in “Consideration Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Person” (2003) from The Congregation for the Doctrine of the Faith, which heavily quoted from “The Catechism of the Catholic Church:”
“There are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God's plan for marriage and family. 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. Under no circumstances can they be approved.’
Sacred Scripture condemns homosexual acts ‘as a serious depravity... (cf. Rom 1:24-27; 1 Cor 6:10; 1 Tim 1:10). This judgment of Scripture does not of course permit us to conclude that all those who suffer from this anomaly are personally responsible for it, but it does attest to the fact that homosexual acts are intrinsically disordered.’ This same moral judgment is found in many Christian writers of the first centuries and is unanimously accepted by Catholic Tradition.
Nonetheless, according to the teaching of the Church, men and women with homosexual tendencies ‘must be accepted with respect, compassion and sensitivity. Every sign of unjust discrimination in their regard should be avoided.’ They are called, like other Christians, to live the virtue of chastity. The homosexual inclination is however ‘objectively disordered’ and homosexual practices are ‘sins gravely contrary to chastity.’”