I suspect that the word marriage picked up its religious connotations during a time and place (Middle Ages, Europe) when religion was the state, and most marriages were religious ceremonies, even if not performed in a religious building.
But regardless of the origins and history of this particular word (after all, there were lots of other words for marriage before marriage!), religion definitely does not hold propriety over the word. I can prove this point with a simple example: Every year, millions of non-religious marriage ceremonies are performed for heterosexual couples, and as far as I know no major religion has been complaining about that.
Since it is acceptable and normal by both religious and non-religious people to refer to non-religious heterosexual marriages as marriages - which are, in effect, civil unions (as opposed to religious unions) - then I don’t see why we would need a new word for civil unions or gay unions.
Almost by definition, the state should not have any problem with the so-called semantics of the word, since that would be a kind of state discrimination. By the same token, there might very well be some religions that are okay with gay marriage, and there could conceivably be religious marriages for gay couples. Religions are not supposed to decide what is equality under the law. Of course, the issue is more complex then that, because we do allow religion to mix with politics. And let’s not forget to mention that it’s not only religion that objects to gay marriage; it’s also basic prejudice against gays.
The argument over the word marriage for gay couples is nothing more than an indirect way to express a person’s disapproval of such marriages. I would like to see a survey to see how these people would feel if gay people were allowed to have, say, civil unions, with ALL of the same legal rights and responsibilities of marriage. Would they really be okay with it as long as the word marriage wasn’t used? Maybe I’m overly cynical, but I doubt it.
For example, one of the concerns voiced by opponents of gay marriage is that if such marriages were allowed school children could be taught or told that gays can get married, which supposedly teaches children that being gay is normal (i.e. If it’s legal, it’s okay). Well, if gays could get a civil union that is the equivalent of marriage, then teachers could tell children that gays can get civil unions. In children’s minds, how much of a distinction would they make between the two if they are really the same thing, and would opponents of gay marriage be okay with this? After all, we’re only talking about the difference between two words that are virtual synonyms. And would this small distinction in any way affect what we call non-religious heterosexual unions? Would they also be called civil unions, and the word marriage reserved only for religious unions?
Of all the things to worry about! A mere word! Is it the word or the substance that is the real problem? If I were gay and wanted to get married I would call my opponents’ bluff and settle for civil unions. I think that taking away that superficial objection would expose the real prejudices underneath.