No, that is not right. Sacramental marriage requires divine sanction. A marriage before a civil magistrate is not a sacramental marriage. Even I know that.
I thought you were asking about Catholic recognition.

The Church recognizes as valid the marriage between two non-Catholics even by a justice of the peace. The Church recognizes as Sacramental the marriage between two non-Catholic Christians even by a justice of the peace.
Not so sure about this. Maybe if two Christians are married in the ceremony of another Church.
That would be valid but so would two non-Catholic Christians married in a civil ceremony. The only civil marriages involving non-Catholics (without impediments) that the Church doesn’t recognize, I believe are those of the Orthodox. Since the Orthodox require a Church wedding for validity, the Catholic Church abides by that.
I think that two atheists that got hitched at the courthouse are not considered to be married and have to have their union blessed.
You might not consider them married but the Church does. It’s a valid, natural marriage. Natural meaning non-Sacramental. Their marriage would become automatically Sacramental when they were Baptized (as Catholics or Protestants).
I may be wrong on this point, my canon law is a bit rusty.
Yes.
Well, we agree on something.
No, those marriages do not have a fixable flaw. Someone with a prior existing bond can’t undo that. There is the polite fiction of the annulment, but surely you agree there are at least some marriages that can’t be annulled (otherwise its just “Catholic divorce”). And defects like perpetual impotence can’t be fixed either.
Prior bonds can always be resolved with the death of the first spouse. Annulment is also a possibility. Permanent impotence, you would be right about but that’s not “literally millions.” And yes, there are a few people who are too mentally ill to validly marry. So, there are very, very few civil marriages that are both invalid according to the Church that could not become valid marriages.
I agree that the Church views all homosexual relationships as “disordered,” but the Church also finds adulterous relationships to be disordered, and considers all second marriages to be a perpetual state of ongoing adultery. So I don’t see the difference.
True which is why no Catholic could support someone being an “out and proud” adulterer any more than they could support same-sex marriage. But, at best, that equates homosexual attempts at marriage with other sinful actions. It does not make a case for regularizing SS"M" or adultery, for that matter.
As to your final point, are you seriously saying that we have to have a whole other set of laws and rules for gay marriage because other kinds of “disordered” relationships are easier to hide? I don’t think that either religious or legal principles should turn on appearances rather than substance
No, I am saying that for many illicit heterosexual marriages, there is no reason to assume that there is something wrong going on. We are supposed to see every situation in its most charitable light. The guy with his third wife
might have received two decrees of nullity. The couple who get married on the beach
might have received a dispensation from the Bishop. That couple who ran away to Vegas and got married by “Elvis” ten years ago
might have had their marriage convalidated since then. But the two men or two women who try to get married **always and every single time **are doing something immoral. There is no way to see that situation is a positive light.
Your question was why is it different. My response answered that question.
I am not talking about laws. I am talking about the question in the OP of why Catholics can’t support or tolerate SS"M" even if it’s just a civil “marriage”.