The word 'marriage' itself in a secular country is not important to me.
But your implied dualism is simply false, an erroneous construction.
Marriage, which is between a man and a woman as husband and wife, began a little before the agricultural revolution 12,000 years ago, and is a global human endeavor that
predates religion, the Bible, Christianity, every religion, and the oath of marriage is taken both in and out of religion, worldwide today, between a man and a woman as husband and wife.
Your "secular country" implication, attempting the inapplicable association of marriage with religion, is simply erroneous with respect to fact.
Thus your implication that marriage is a religious institution, an erroneous implication, is meaningless.
A civil union would be just fine as far as I am concerned.
A domestic partnership civil union is exactly what marriage -- between a man and a woman as husband and wife -- is in the government's eyes.
Domestic partnership civil unions can be more than one type, marriage simply being one type. Gays in romance can have
homarriage, polygamists in romance can have
polmarriage, all can be specifically and similarly defined domestic partnership civil unions, with, of course, different names to intelligently differentiate, just like cat-owners have
cat shows and dog-owners have
dog shows, to keep within the intelligent application of accurate naming according to definitive propriety.
There will be some who will argue against polygamy, and that will be a battle, I suppose, though I have no "dog" in that fight.
My beef is in doing stupid things and then legally compelling society and citizens to respect such stupidity, like ludicrously associating marriage with gays and polygamists and the like, as that's simply a violation of definitive propriety, and such stupidity dumbs down subsequent generations and creates societal regression as opposed to progression.
Although, I don't see how it preserves the sanctity of marriage from your POV provided that (read IF) the SC allows for gay marriage.
A SCOTUS pre-conceived ideological mistake of allowing SS couples an association with marriage, not requiring a separately named domestic partnership civil union for them, would simply be a mistake, an egregious violation of definitive propriety, complete politically motivated.
It will still be a mistake, even if it's the SCOTUS making it .. and, it can be reversed in time, but in the meantime it would just be a step back, as in "two steps forward, one step back", the typical way our species progresses.
Again, your use of the word "sanctity" in describing marriage is to imply a religious connection, which I've pointed out is an erroneous assumption on your part.
Marrage is a time-honored human institution, predating religion, the oath of which is performed outside of religion as well as adopted by religion for religion's control sake, and thus religion is meaningless in the matter.