The problem is, once you use the term in government it is no longer JUST a religious term, but a dual term.
There is religious marriage and then there is governmental marriage.
Marriage, in the government, CAN'T be religious because if it is...and we start upholding it to the traditional religious meanings of it...then we are choosing one or multiple religions over others and forcing it on someone which is the very meaning of establishing a religion through law.
So whatever idiot however many years ago that decided to intertwine the word is at fault.
The best option is complicated, but yet simple at the same time.
Strip the term marriage from all law, entirely. Put it in a word file, hit ctrl+f, go to replace, search for marriage and set it to replace with civil union.
Seriously. Swap it out. Make ANY union between two people recognized by the government a Civil Union.
So then the religious people are able to have the term "Marriage" completely to themselves and if they don't want to let two men or two women get married, they're absolutely free too.
So then there is equality under the law, because two men, two women, man and woman, friends, lovers, soul mates, whatever, can all enter into civil union
Boom, simple, be done with it. If you have issues still with that and are trying to talk about the sanctity of marriage then you're just absolutely spinning and are being dishonest, because that would keep "marriage" sacred and you're just really upset by the notion that two men are allowed to file joint taxes and have the ability to make medical decisions for each other.