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?
You could argue that there are laws that only apply to married couples, and that brings meaning to marriage. Firstly, generally speaking, things we associate with marriage culturally are mostly not protected by law, most notably: fidelity. 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.
Why is the reaction seen as irrational or immature but not the action that triggered it?
The analogous (but with an opposite direction) action would be campaigning to make gay marriage legal. Nobody has a problem with people doing that. The reason people object to Eich's firing is because it is a very clear escalation in the culture war, not because they have strong opinions about gay marriage.
Clearly being black, or hispanic, or asian, or white are physical characteristics. Far fewer people would argue that there is any element of choice in that.
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.
The main benefits are tax free gifts between partners and filing jointly, both of which seem very reasonable and wouldn't be of value to single people.
The actual tax breaks most people think about are tied to dependents in your household, not marriage.
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.
I don't think childless couples (of any gender) should get any societal advantages yet I have no problem working with people that disagree. Why has everything to be black-or-white, left-or-right, with us or against us? That's not a productive way to think about others.