This argument is ironic, since those who make this claim often make rather cynical and disgusting judgments of others based on their political positions, number of children, personal appearance, or other accidents (Check your average Facebook feed or comment box).
But it is also inaccurate, since Christians are specifically enjoined not to "judge" individuals based on personal choices. In fact, Christians aren’t supposed to “judge” people at all. We can’t know the eternal fate of anyone (except those canonized by the Church, who are in heaven) or the state of anyone’s soul, or how "good" or "bad" anyone is at any given point in time.
But Christians can and should judge behavior – or perhaps more accurately, we can say with certainty that certain behaviors are clearly wrong, as judged against a pre-existing standard. Insofar as Christians “judge,” they judge the actions of others based on the unchanging moral standard of the moral law of God.
Christians have a duty to warn others that certain actions are indeed evil – especially in a world which celebrates certain evil actions and enshrines moral relativism as its guiding philosophy.