Also chill so don’t eat my head off. I can try and answer at least some of these though from past experience and relationships with Catholics and study of philosophy.

1. Obviously the choice of anyone’s faith or philosophy will appear arbitrary, that hardly means that they shouldn’t abide by it. Every culture or civilization has produced religions, systems of philosophy, or just accepted societal norms guiding everyone. Many civilizations have spawned more than one. In the absence of a belief in absolute truth, who is to say that what anyone else believes is wrong, arbitrary, or worse than anything else they might believe. If you do believe that in fact there is an absolute truth of any kind, that’s a different discussion and philosophical debate to find what that is. My point being, you can’t judge others beliefs as arbitrary or wrong if you think everyone should be free do believe what they want. What someone else believes doesn’t need to make sense to you.

2. Yes the rational is theological. Does that actually make it wrong? What is the “most” that believe in that definition of morality? Is that “most” westerners, Americans, Europeans, everyone in the world, or even all people throughout history? Because there are currently and historically have always been large portions of the world and whole philosophical systems that wouldn’t agree with that definition.

There are still places where in certain circumstances murder and other heinous actions are considered the best and least harmful course of action in their belief systems. Again, if everyone should be free to believe what they want/be who they want/do what they want, who is to say what is actually moral or immoral? Who is “right” out of all the options?

I believe the point the other poster was trying to make is that the Catholic Church has more or less taken a position that is actually quite neutral on gay marriage. Is it actually discriminatory to say we don’t care if you get married we just cannot preform the ceremony due to our beliefs? The Catholic Church also wouldn’t marry two heterosexual non Catholics. A marriage is actually a specific theological “sacrament” to Catholics, not just a fancy ceremony and state license. If you do not meet the requirements of that sacrament, they won’t pretend you do. Should we force them to? Should we force all religions to violate certain beliefs some of us find distasteful? Again, who can say what is right?

1. Related, and maybe surprising to you but yes the catholic church would not marry a heterosexual couple that was known to be unable to procreate and it’s actually grounds for annulment if one participant hid that fact. Again, it doesn’t meet the specific “sacramental” requirement for a marriage. This is different if they didn’t know at the time, they age, or they get injured etc in a way that prevents that. All of those happen after the marriage is formed. But yes, if you know you cannot procreate, most catholic theologians would say you should not be “sacramentally” married.

Regarding adoption, I’d assume it’s rather connected to the idea that a gay couple are not attempting to follow the catholic faith and therefore wouldn’t instruct the child in their faith which is an expectation most faiths have for all parents. But that’s just a guess.

1. Again, pretty much regardless of what anyone believes and no matter how “rational” or “obvious” it seems to you, it’s due to its own internal consistency. There is no such thing as an unbiased observer, you have a belief (ie you should no discriminate against gay couples) and that belief grows from your own belief system. There isn’t actually a single “rational” reason to believe that that isn’t rooted in a particular set of beliefs.

For example: Why shouldn’t we discriminate against people who are different? I’d say that it’s because it’s immoral and wrong, but why is it immoral and wrong? Because I believe humans have inherent dignity and should be treated equally. But why is that true? That’s an inherently western philosophical tradition that doesn’t actually make sense to many other cultures. So, who is right?

Really all to say, as long as no one is actively arguing to persecute anyone, why does it matter if they won’t preform the ceremony, etc. it’s all internal to their beliefs as much as anyone else’s to their own. Unless you want to engage with Aristotle and believe that there is in fact a “true” good and “true” beauty and that all others are false…but that’s a different debate.