Honestly, I think the government should get out of the marriage business entirely. Separate but equal does not work for different people; separate but kind of similar does work for church and state.
Call it "marriage" for church, if you want. My nominal faith will probably call it "handfasting". Call it "Legal Union" if you want it recognized by the government.
It's what we computer people call "overloaded": the same word is used to mean different things depending on context. It's elegant to overload a function when it's very clear by the inputs what the overloaded function is going to do (I believe that a common one is to use the "+" sign to both add and concatenate, so numerical 2 + 2 = 4, but character "two" + "two" = "twotwo") but it's the mark of a fucking kludge when the user has to spend half an hour sorting out with the support people exactly what they're putting in and expecting to get out, and it's non-obvious what's going wrong unless you're leaning over the user's shoulder and watching their keystrokes and know what's going on inside that fucking black box.
"Marriage" is currently used to mean:
- the package of legal rights and responsibilities for a legally wedded committed partnership
- the package of legal rights and responsibilties for a common-law committed partnership (in states that support that)
- the package of social expectations that go along with socially acknowledged committed partnership
- the package of religious rights and responsibilites that go with a religously-sanctioned committed partnership
No one should question the right of any given person or religious organization to deny their personal religious support to same-sex committed partnerships; this is up to the individual or group's intrepretation of their holy texts (if applicable) and their relationship(s) with their deit[y/ies] (if applicable). So long as their approval or lack of same does not break any laws (stoning to death is legally frowned on; public nudity is also often legally frowned on in certain areas), what a religious person or group has to say about any practice in religious context is up to them.
The right of members of society to deny their social support to committed same-sex partnerships may be soundly booed and razzed, but it should be a protected right. My fictional conservative Great-Uncle Morton is not legally obligated to attend my wedding to another woman with a smile and a gift, nor is he obligated to look upon me kindly when writing up the distribution of his material possessions after his departure via death. (My fictional doting Great-Aunt Gillian may socially force him to attend the ceremony and may make compensatory provisions in her own will, but that's another story.)
The legal position of a couple who has established a tradition of sharing housing and other resources, but have not gotten joined via paperwork is an iffy one. Some areas support joining them legally, automatically, after a certain term, or after certain informal criteria have been fulfilled (I am thinking specifically of something I heard, that if you sign into a hotel as married partners, if you weren't before, you become so). Other areas do not. This is the form of marriage I know least about.
Finally, we have the legal committed partnership, which joins together land, money, and other possessions, makes the partners legal next of kin to each other, makes automatic arrangments for custody of children (if any) in case of death, and so on and so forth. Hospital visitation. Inheritance without penalty. Often, insurance coverage. This is the bundle of legal rights and responsibilities that opposite-sex couples have if they wish to pursue it (barring any legal barriers from obtaining same, such as a previously existing exclusive contract of the same sort, or financial barriers such that if the couple were legally joined, their finances would be flushed down the same crapper) and that same-sex couples wish to have easy access to. An opposite-sex couple can sign paperwork, and have the process over with on a drunken impulse while visiting Vegas. There is nothing of the sacred, and much of the legal and binding, about the government-endorsed act of marriage. The only sacred that happens is if the participants wish there to be some, in which case it is covered by the separate religious meaning of the thing.
Corporate entities (insurance companies, banks, and their ilk) tend to observe legal contracts such as legally recognized marriage and domestic partnership, but ignore social and religious bonds. My ex-fiance BJ and I were religiously married, but never legally married. When making full accounting of my liabilities to a spiritual partner, I must include my breaking of/release from a spiritually binding marriage, but the Federal Application For Student Aid could care less, unless there was paperwork signed to start the marriage, and divorce or anulment papers signed to end it.
By all means, if marriage is an important religious sacrament to you, insist that government get their paws out of it. Insist that if your children are to have their union blessed and recognized by you, that they must, in addition to the legal paperwork, have a religious ceremony in an establishment of religion of your choice. Feel free to extend or deny religious marriage to anyone you and your establishment of religion choose to.
It's your choice, and that's one of the fundamental freedoms of this country, that if I'm not of your religion nor affected by it, then I have no say in the internal rules save that they follow the laws of the country (not harming innocents, not formulating weapons of mass destruction, not forging currency or stamps, et cetera). Likewise, religious groups must accept that their laws are not the laws of the country, and that to keep their law in addition to the country's laws, they must instruct their people in their own rules and not ask that others not of their faith follow their rules.
I am allowed to eat bacon cheeseburgers and work after sunset Friday nights and all day Saturday. By the rules of this country, I may, and by the rules of my faith, I may. mamadeb is allowed to eat what she pleases and work any hour that she pleases by the law of this country, but by the law of her faith, she may not eat that nor work then. I do not insist that she break with her faith; she does not insist that I comply with hers. Of courtesy for her, I would probably refrain from eating a bacon cheeseburger in her presence, but that is my social choice. Of courtesy for her, I would probably ask if there were any task that she wishes could be done on the Sabbath; this is also a social choice.
Social law, courtesy/politeness/custom, is the lubricant that fills the gaps between secular and religious law, and laws from differing religions. If I were to choose permanent religious and legal partnership with another person, I would have a legally recognized officer of legal marriage of our choice do up the papers, with us and witnesses of our choice. We would likely have at least one religious ceremony, possibly two, especially if we come from different religious backgrounds. By preference, we might not have particularly many guests at either the legal or the religious ceremonies. We would probably have a relatively good-sized social ceremony, with friends and family invited. I would not ask fictional Great-Uncle Morton to watch someone tie our wrists together and then watch us jump over a broomstick or a fire (though I might invite Great-Aunt Gillian); and I have a few friends who I suspect would be glad to avoid any ceremony that takes place in a building with lots of stained glass and crosses. I would happily ask Great-Uncle Morton to join us for cake and punch and dancing, and I'd probably make sure that there was a version of the goodies he could have, what with his diabetes and all, and I would try to make sure that he was seated well away from the slashgrrls.
One reason I suspect that the government is reluctant to endorse a union between more than two people at a time is because, in case of dissolution, the red tape of whose stuff is whose is going to be a bloody nightmare, and I don't blame them at all for that. It's bad enough with just two.