Grace & Peace!
The difficulty in most discussions like those mentioned in the original post stem from the two sides of the debate not actually talking about the same thing. Here’s what I mean:
1–The RC Catechism does not once, to my knowledge, mention the words heterosexual or heterosexuality, and it’s discussion of homosexuality is couched in terms of psychology and disorder. However, it views sexuality very positively and discusses its importance in human wholeness. It’s clear that, while the catechism does not come out and say so directly, Rome does not believe in sexualities. That is, it does not mention heterosexuality because it does not believe in it, and it does not believe in it because doing so assumes that heterosexuality is a variety of human sexuality. Rome believes in one fundamental human sexuality, oriented toward marriage, finding its most complete expression in chastity (either within or without the married state). As such, homosexuality is a disorder of a primal (as in original) sexuality. It is an illness and is not reflective of a rightly ordered and/or healthy sexuality. The difficulty in terms of the catechism, is in explaining how the inclination is a disorder but not a sin, and the acts are disordered but sinful–that is, it is possible to have a disordered sexuality but not be in a state of sin, even though an ordered sexuality is important to human wholeness . Difficulty, however, attends all forms of nuance and Rome’s position on homosexuality does not exclude nuance.
2–For many others, sexuality is a fundamental aspect of human wholeness which is legitimately expressed in a variety of ways–heterosexuality, homosexuality, and bisexuality are all viable expressions of human sexuality which relate to our capacity for relationship (much as the primal sexuality of the catechism relates to our capacity for relationship as well). These sexualities can become pathological if they are expressed coercively, violently, obsessively, inappropriately, or non-consensually. Otherwise, they are not generally seen as pathological or in need of healing.
Because a similar vocabulary is used, these two camps’ understanding of human sexuality are appear similar enough to make people believe they’re talking about the same thing–they’re not. Camp 1 speaks of a singular sexuality that can be corrupted. Camp 2 speaks of sexualities that can be corrupted. This presents a very particular challenge to any Roman apologist. This stems mostly from the fact that no truly respected psychological or medical institution or organization agrees with Rome’s fundamental point–that varieties of sexuality do not exist as such.
For the most part, therefore, many people will hear a Roman apologist’s statement that homosexual acts are sinful and the homosexual inclination is disordered as saying, “in order to be good people, homosexuals must be sexually repressed and view their sexuality as a source of sin, hating it all the while. It is impossible for a homosexual to properly relate to anyone (particularly sexually) given a fundamental deficiency in their sexuality. Homosexuals are not quite human, therefore–or, in order to be human, must deny a fundamental aspect of their humanity. But it’s all a mystery that I can’t quite understand because the magisterium tells me what to believe so I don’t need to bother so much with understanding things.” Clearly, this does not sound like a very loving thing to say and is even quite offensive. And while I, admittedly, do not agree with Rome’s fundamental position, this is not quite what a Roman apologist is actually saying. But that’s how its heard.
When a person from camp 2 talks about sexuality, however, someone in camp 1 is likely to hear, “it’s okay to do what whatever you want, particularly sexually, because sexuality is a good and sexual expression is important and as along as it doesn’t hurt anyone, everything should be fine. You can do what you want. I’m clearly delusional and that’s okay too because I’m a moral relativist and today is Tuesday so delusion is fine today.” It sounds a bit ludicrous. But it’s also not quite what the camp 2 person is saying. But that’s how its heard.