I apologise for seeming abrasive, I think if you think something is wrong there is the tendency to just come out and say it.
I see the same in reverse and I think I am just ‘reacting’ to that. For example your own post just says things as though they are objectively true and to me they seem twisted around - the argument that homosexuality is against natural law would need to be shown - not the other way round. Homosexuality pre-dates the catholic church.
To say that sin is not a matter of opinion, I don’t understand as religious sins are not automatically enacted in law.
If something is prohibited we have made a law against it and some ‘sins’ are laws e.g. murder . But not all ‘sins’. A sin to my knowledge is something that goes against ‘divine law’ so apostasy in terms of islam is a sin from god to the muslim world, but that clashes with the catholic view - as a catholic cannot be a muslim - which is the correct sin. As I see it sin is open to interpretation and what one accepts
The natural law argument can be found here:
catholic.com/tracts/homosexuality, and in more detail in other places. What I meant to imply was that there was an argument which Catholics hold to be correct, and some of us will debate it with you if you desire, but that to debate this issue with us requires approach along one of those lines to be very lively (or perhaps from some other direction I haven’t thought of, but it usually boils down to one of those).
Often we will state Church teaching as fact here, because as Catholics, we believe it to be so and the fact that the Church teaches it
is an argument among Catholics. It is the default Catholic position that what the Church teaches is correct. We will be happy to debate the whys and hows, but when just answering a question what we will tend to do is say what the Church says, with the understanding that the argument for its correctness is based on the authority of the Church (which we will also be happy to debate, where it should be appropriate). However, we will often debate the issue if an objection is raised, or some reason is given as to why a poster thinks it is incorrect.
Sin is not equivalent to civil law. Not all sins should be outlawed, and not all things which are outlawed are necessarily sins in themselves when the civil law is not considered (driving over 35 on a certain road, for example). There are relationships that should exist - if a thing is truly wrong, then perhaps that particular thing should not be prohibited by the civil power, but it is prohibited by God, and the civil power should not encourage it or call it good. But the fact that laws can be passed one way or the other does not say much for what is and is not sin (though what is and is not sin does say something for which laws
should be passed).
The fact that many people disagree over what sin is does not relegate it to a matter of opinion, no more than the fact that in the recent past physicists were not sure whether the universe would expand forever, or contract again to a single point made that a matter of opinion. One of the two (or something else)
will happen, whatever we think. We can try to discover which is true, but it is not something we decide, it is not a preference.
Many people try to approach the question of what is sinful and what is not in different ways, and many people get different answers - but all that means is that many people are wrong, not that the question is on the level of “is dark chocolate or milk chocolate better”. (Milk of course because… ahem.)
But as far as Catholic goes, we get our answers from Church tradition, scripture, and natural law, and to debate a position that Catholicism holds on sin requires an approach from one of these angles.
In general, though, natural law is a good place to start for those who don’t believe in revelation through scripture and tradition, as we tend to think that the natural law contains all of basic morality.