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.
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.
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).
Yes.
Well, we agree on something.
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.
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.
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”.