Whether marriage should be considered a religious institution has consequences for how it should be legally defined. Marriage is certainly an important part of a religion like Christianity and the Christian Bible prescribes how Christians should go about it, but non-Christians marry as well, and people of other faiths are bound by different sets of commandments concerning marriage that are incompatible with the traditional teachings of Christianity. In Islam for instance, a man is technically permitted to take up to four wives so long as he treats each of them fairly (Qur'an 4:3). A Hindu marriage isn't preceded by courtship or even love, but arranged by family to avoid the blinding influence of infatuations. And if we open our eyes completely, we can't help but notice that non-religious people are in the habit of marrying as well, which means religion's claim on the institution of marriage appears rather dubious. But if marriage isn't specific to any one religion or to religion at all, then which (if any) religious values should be reflected in the laws governing it?
The only way to protect religious freedom is to enact laws that are neutral with respect to the manner in which marriages are carried out. This means that no ban should be imposed that would prohibit a practice sanctioned by a religion. A ban on same-sex marriages for instance would prevent any religious denomination that sanctions them such as the United Church of Christ from carrying them out. On the other hand, if same sex marriages were legal, it wouldn't force denominations that disapprove of them to carry them out any more than it would force them to allow polygamy or any other non-Christian forms of marriage. A ban on same-sex marriages is therefore not, as it is often presented, protecting religious freedoms, but quite directly attacking them.
Due to the offence that a same-sex inclusive definition of 'marriage' causes to certain religious sensibilities, one solution that has been adopted in various countries such as the UK and Australia has been to grant homosexuals the right to enter into a legal union that goes by a different name. These 'civil unions' confer partners in same-sex relationships the legal status as next of kin, which has consequences for how individuals are regarded when visiting their partners in hospital, how inheritance and custody issues are dealt with when one of them dies and so on. In many jurisdictions, civil unions confer all of the same rights that are enjoyed by married couples, just under a different name. As they say, if something looks like a dog, smells like a dog and barks like a dog, it probably is a dog, but when it comes to same-sex partnerships, there are those who insist that the institution that looks exactly like marriage and which confers all the rights of marriage isn't a 'marriage', but a 'civil union' and should be distinguished as such under the law. Put that way, it's surprising that anyone cares one way or the other whether these partnerships go by the same or different names, unless of course we acknowledge that this issue is acting as a proxy for a separate and much more fierce debate. The underlying conflict is of course over the morality of homosexuality itself, but it's being waged under the more widely palatable cover of concerns about religious freedoms and quibbles over the definition of 'marriage'. So the hostilities find a crack through which their flow is tolerated, except there's so much pressure behind them that we find a torrent where we'd expect a trickle.