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.
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.
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.