There is a difference between judging people and judging actions.
Judging people can be problematic on many levels because we ordinarily do not know an individual person's level of personal culpability for the actions he commits. Generally speaking, judging people is best left to those invested with the authority to do so: God and those to whom God has delegated a portion of his authority through the Church and through the civil state (e.g., priests, civil judges).
Judging actions is something that everyone must do, using his properly formed conscience. The Catechism of the Catholic Church
has this to say about the role of conscience, which it both upholds as man's first teacher and insists must be rightly shaped through knowledge of and obedience to authoritative teaching:
Conscience is a judgment of reason whereby the human person recognizes the moral quality of a concrete act that he is going to perform, is in the process of performing, or has already completed. In all he says and does, man is obliged to follow faithfully what he knows to be just and right. It is by the judgment of his conscience that man perceives and recognizes the prescriptions of the divine law:
Conscience is a law of the mind; yet [Christians] would not grant that it is nothing more; I mean that it was not a dictate, nor conveyed the notion of responsibility, of duty, of a threat and a promise.... [Conscience] is a messenger of him, who, both in nature and in grace, speaks to us behind a veil, and teaches and rules us by his representatives. Conscience is the aboriginal Vicar of Christ (CCC 1778).
Conscience must be informed and moral judgment enlightened. A well-formed conscience is upright and truthful. It formulates its judgments according to reason, in conformity with the true good willed by the wisdom of the Creator. The education of conscience is indispensable for human beings who are subjected to negative influences and tempted by sin to prefer their own judgment and to reject authoritative teachings (CCC 1783).
In the case of homosexual marriage, while it is possible that individuals engaging in it may have greater or lesser degrees of culpability based upon their personal level of knowledge and consent; it is certain that the proposed action itself is impossible. Just as only men can be priests, so too can marriage only be formed between a man and a woman:
The Church's teaching on marriage and on the complementarity of the sexes reiterates a truth that is evident to right reason and recognized as such by all the major cultures of the world. Marriage is not just any relationship between human beings. It was established by the Creator with its own nature, essential properties, and purpose. No ideology can erase from the human spirit the certainty that marriage exists solely between a man and a woman, who by mutual personal gift, proper and exclusive to themselves, tend toward the communion of their persons. In this way, they mutually perfect each other, in order to cooperate with God in the procreation and upbringing of new human lives (Considerations Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Persons 2).
Special Report: Gay Marriage
Early Teachings on Homosexuality
Considerations Regarding Proposals to Give Legal Recognition to Unions Between Homosexual Persons
by the Congregation for the Doctrine of the Faith