Even if we can't apply religious doctrines in our laws per se, we do not want to force Americans to pay for, or honor practices which violate their beliefs.
And as I've said, the only way to accomplish that is to make taxes voluntary. Tax funds are simply not distributed in accordance with the religious beliefs of the taxed. There are people who oppose military force of any kind for religious reasons; nobody makes sure their specific tax dollars don't go to the DOD, or even that tax dollars are allocated to the DOD in proportion to the number of nonpacifists among the taxpayers. I spend a lot of time in a local national park, paid for in part by the tax dollars of people who don't believe the government has any business maintaining such lands. You can think up lots of other examples yourself, I'm sure.
There will always be problems of this sort as long as there are taxes; marriage isn't a special case. (Similar problems infect public education, which can't possibly be performed in a way that pleases everyone who pays for it; the only way to change that would be to privatize the schools.) Either privatize everything -- which in this case would mean getting the state entirely out of the business of recognizing marriages -- or get used to the fact that there's always going to be somebody paying for something of which they disapprove.
This is why I initially suggested that you keep in mind the question whether any positive legal benefits should be conferred on civil marriages at all.
If marriage is defined as a sacred and traditional bond between man and woman based on religious doctrine common to most Americans and western civic tradition common to most other cultures, it really has no bearing on other relationships. If we change its meaning on our lawbooks, it still becomes something else.
Then I'm afraid I have bad news for you: it's already become something else. Civil marriage is not defined in terms of sacredness, tradition, or religion.
Religious marriage is defined in such terms. And private religious organizations don't have to recognize civil marriages as 'religiously satisfactory', as for example the Roman Catholic Church doesn't have to recognize civil marriages or divorces for purposes of its canon law. Civil marriage is already not the same thing as religious marriage.
You'll note well that I wouldn't be here, associating with my fellow right wingers if I too strongly disagreed with them. Usually it's a semantic argument that I'm having with their justification for various positions. I'd sooner blame their utter frustration with post-modernism than any particular bent on infringing on others' rights. Like them, I recognize the legal authority of various American officialdom, but I reserve the right to withhold my approval. We hold certain truths to be self-evident. Mother nature can teach us much more about marriage than a supreme court justice. The same-sexers want something from us that we don't want to give. They don't need it, it's not listed in the bill of rights or even covered under the "pursuit of happiness." But still they want it, they crave it.
The question I have for you is more fundamental. Would you rather have justices appointed who know what I'm talking about, or ones who prefer the Roy Moore point of view. The choice is the left's. The more they press, the more religious the right will become. It's a simple physics: for every action, there is a reaction.