Many North American Christians have improperly formed consciences. This isn't to say that people are wicked - they almost never are - but it is to say that many of us don't engage in moral reasoning in a way that is consonant with the Christian tradition.
Instead of thinking with the Church, that is with the saints throughout the ages, we think "for ourselves." We often take great pride in this. But we don't really think for ourselves do we? What generally happens is that Christians end up thinking pretty much like everybody around us. We don't hold to Christ's view about a moral issue, or even come to our own conclusion. Instead we make our own whatever is the popular sentiment (One hesitates to use any term that would suggest more than a mere feeling) about the matter.
For many North American Christians, the words quoted sound harsh. And yet the ability to distinguish between the sin and the sinner, or between the act and the actor, is what prevents us from being identified with our failures (or for that matter, our successes). Put another way, the distinction the authors draw reminds us of the primacy of the person, and so of love, in Christian morality.
Unfortunately the primacy of the person - and so of love - is closed to those who reduce personal identity to ideology. Whether that ideology is, as in the quote, sexual, or political or economic doesn't matter. An adjective - at best - reveals only an aspect of a person. When identity becomes absorbed by a qualifier the person in her uniqueness is lost. Further because we are created in the image of God our unique, personal identity is always a mystery to us known fully only to God. Because of this we are always tempted to short-change ourselves, to ignore the mystery of our own identity.
Strictly speaking, my self-knowledge is always relative. This is why like the dogmatic tradition, the moral tradition of the Church is essential. The Christian tradition teaches me to weigh properly - and to express rightly - the different aspects of my personality.
More than this though, the tradition helps me to integrate the often conflicting and disparate elements of my personality. Doing this is also essential for me to be able to respond in a healthy manner to the wide range of demands life makes on me. What I'm describing here is the life of virtue. For the Christian the virtuous life, is the life in which the person has successful integrated the different elements of personality in light of the Gospel.
Virtue also is dynamic because my life is dynamic. This is why, after divine grace, a life of virtue is dependent on a rightly formed conscience. It is rightly formed conscience, that is a conscience conformed to Christ, that gives me the ability to evaluate in light of the Gospel myself and the constantly shifting demands life places upon me. Apart from a rightly formed conscience, I cannot acquire (and so cannot live) a life that is psychologically integral and in harmony with my circumstances. To re-work an old phrase, "No virtue, no peace. Know virtue, know peace." It is a rightly formed conscience, that is a conscience formed according to the Christian moral tradition after the example of Christ, that allows me to know virtue and so acquire "the peace that surpasses all understanding" (see Philippians 4:6-8).