Marriage is a formalization of recognition of a biological fact of life wrought by millions of years of evolution. Only luddites and flatearthers could imagine otherwise. So, no, it’s not originally a religious practice.
Marriage is a religious concept. The grouping of one make and one female is biology for the raising of off spring.
In either case the government has no business re-defining either.
As I read your post, you sound like a Libertarian. Marriage pre-exists the state. If you are a conservative and not a Libertarian on marriage, would you please answer the following:
Why does marriage require the state to define it, regulate it, tax it, subsidize it, give married persons certain rights and obligations not available to non-maried persons?
Why aren’t these matters left to the parties involved?
What is the state interest in, for example, giving a larger S.S. benefit to the survivor of a marriage in which the spouse worked regularly, than to the surviving partner of a gay relationship?
And, what is the state interest in giving two S.S. benefits to gays in a relationship when both worked, and giving only one S.S. benefit to a married couple when both worked?
Or, do you think all or most of these discriminations were never intended, and have simply accumulated into a really complicated mess over the years?