If there's nothing fundamental about marriage and it's just some weird coliving arrangement, then why ban it for only some groups in the first place? Nothing productive or even rational about it.

Why is the reaction seen as irrational or immature but not the action that triggered it?

It has to be us vs against us because that's what law is all about -- outlawing certain actions.

It's one thing to believe as you do, it's quite another to push for legislation that would (in your example) deny childless couples societal advantages, whatever that actually means.

If you're not in favor of a-or-b arguments the answer is to allow a and b, eh?

For one, being childless is a choice (mostly, especially since adoption is a possibility). It's indeed OK to have different opinions for what how laws apply differently to people based on their choices. Being gay is not a choice, it is rather similar to race/ethnic background, and it's generally not OK to have laws that treat people differently based on something like that. I'm sure there are more nuances to add, but it seems to me that makes it quite a different situation.

Your thinking applies equally to all people. His donation tries to take away a right from a minority group. They're quite different.

In a liberal context, marriage means nothing except for being a symbol of a union between two people. But all rules, obligations and rights that make marriage a meaningful institution are rooted in religion, and are hence not always respected outside of religion.

You could argue that there are laws that only apply to married couples, and that THAT brings meaning to marriage. But:

Firstly, generally speaking, even the most important features of a marriage are not protected by law, most notably: fidelity. So the law is disjoint from what's traditionally considered to be obligations within marriage. That leaves the legal definition at the whims of contemporary polititians. Therefore, law cannot assign the word "marriage" any consistent meaning throughout time.

Secondly, to my limited knowledge, the line between a married couple and two people living together is increasingly getting blurred by laws that apply marriage legal obligations even to non-married couples if they have lived together for long enough. It suggests that law-makers do not consider a ceremony and a "marriage" announcement to be what should really activate these laws, but rather other factors. Although, they seem to acknowledge that an announcement of a marriage implies the factors needed to activate these laws. If that makes sense...

So marriage is inherently a religious institution that in a religious context comes with rules, obligations and rights. Hence why people who take religion seriously will find it offensive that somebody that completely disregards these rules calls themselves married.

If you think for a moment, you'll realise that it's irrelevant whether marriage is important to you - it's important to many.

So when a person wishes to deny this important institution to a minority, they are creating an out-group and discriminating against them.

By that logic, we can put those discriminators themselves in an out-group and discriminate against them. We can deny them institutions such as directorships. Fair's fair.

It's more than fair - despite what conversion camps want to sell, being queer is an intransigent characteristic. Being a bully is just a choice. Discriminating against bullies is as morally just as discriminating against the incompetent.

Obviously it's fine to campaign against marriage.

What unjust "advantages" do you think childless couples get that you would want to get rid?

Pretty much all of the legal benefits of marriage are contractual, not financial, and come at no cost to the public.

Things like spousal medical rights, a joint estate, etc don't come at the expense of anybody else.