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(Updated 26 June 2015, the day the Supreme Court struck down state bans on marriage between persons of the same sex across the United States.) A common reaction to gay marriage is, “if we legalize gay marriage, what's next?” This ‘slippery slope’ argument was used mostly as an emotional scare, usually with little rational thought involved. However, it's still a good question. Throwing out religious preferences for ‘traditional’ marriage, and starting with an exclusive union between two adult humans as a baseline, there are a few practical ways the legal institution of marriage can be expanded:
One way to incorporate polygamy is to simply increase the number of members a marriage has. To keep things fair, everyone in a group marriage is equally married to everyone else in that group marriage. Formation of a group marriage requires the consent of all members; if one member wants to terminate the marriage, the others can't prevent it, and legal proceedings for separation follow. Really, there's no need for the law to distinguish a marriage between three or more people from a marriage between just two people. Sex among members of a group marriage is never adultery; whether or not adultery is a crime is beyond the scope of this discussion.
Certain legal procedures need to be established: a single person joining an established marriage (consent of all parties required; dissenting parties in the established marriage have divorce as an option); two (or more) established marriages merging to form one group marriage (again, consent of all parties required); separation of one (or more) person from a group marriage (which should have the same end result of a divorce of the whole group followed by remarriage of a subset, but may have a more streamlined procedure); division of a group marriage into two or more marriages (similar to the previous procedure).
Marriages should be subject to age-of-consent and ‘Romeo & Juliet’ laws considering the greatest age difference among the marrying parties. Community property rules applicable in most states should need very little adaptation. If a couple wants to marry, and preserve the binary nature of their own marriage, they may form a prenuptial agreement barring future additional partners in the marriage.
The other way to incorporate polygamy is to make marriage non-exclusive. This is almost certainly the most severe departure from traditional marriage; in fact I hesitate to use the word ‘marriage’ to describe it. What marriage means to the parties involved is considerably different if the marriage is not exclusive. Community property is essentially incompatible with marriage non-exclusivity. For these and other reasons, non-exclusive marriage should almost certainly stand apart from exclusive marriage.
Marriages must be declared as exclusive or non-exclusive from the start. Conversion from non-exclusive to exclusive marriage (or vice versa) should be possible; it should have the same consent requirements and overall legal impact as a dissolution and remarriage, but may have a more streamlined legal process. A person may be a member of more than one non-exclusive marriage, but may not be a member of multiple marriages if any are exclusive. Sex between members of a non-exclusive marriage is never adultery; sex between a person in a non-exclusive marriage and a single person (or between two people not married but having a common non-exclusive spouse) might technically be adultery, but practically should be of no consequence. If non-exclusive marriage exists and marriages may have more than two members, then non-exclusive marriages may also have more than two members, though the necessity for that option may be unclear. In a non-exclusive marriage, each partner has the same legal ability to enter into other non-exclusive marriages, though prenuptial agreements may theoretically provide a disincentive to doing so for individual members.
Laws concerning marriage don't need to require that members are human, only that they are persons capable of entering into legal contracts. As soon as such ‘personhood’ recognition is bestowed upon animals, the debate over human-animal marriage will become relevant. Again, sex between people who are married should never be considered adultery or, at least in a legal sense, bestiality — though you can still call it a sin if you want. Given the current political climate, however, it appears more likely we'll first be debating individuals marrying corporations.
I add this section only because people seem to worry about the normalization of pedophilia. Adult/child romantic relationships are problematic at best. I don't forsee any realistic way to implement this any time soon. Our culture would have to change dramatically before we can even have the conversation as a broad community about how to address the problems with normalizing these relationships systematically, not to mention the intense moral opposition.
No religious institution should be required to perform any marriage not compatible with its views. Inversely, a religious institution should be allowed to perform any marriage, regardless of the legal definition of marriage; if a marriage is performed that does not fit legal requirements, then the partners are not married by law, but the institution which performed the marriage should not be punished either. If a couple or group can't find a church which will bless their union, they would probably be happier with a secular ceremony anyway.
I suppose businesses don't have to recognize the marital statuses of their customers if they prefer to only recognize a limited kind of marriage like one-man-one-woman, or two-men-one-cat. It's discrimination, yes — but so is ladies' night at your local tavern. At what point that discrimination becomes illegal is for the people, legislatures, and courts to decide, but I think free enterprise (except perhaps the insurance industry) should have quite a bit of latitude, at least when it comes to policies dealing with customers.
On the other hand, employers should be required to recognize (at a minimum) the same types of marriage recognized by the government, and offer equal marriage-related benefits to all their married employees — however reduced benefits for non-exclusive marriages might make sense considering the massive potential for abuse. Again, that's up to the people, legislatures, and courts to decide.
The purpose of this document is not to advocate for the radical expansion of marriage as described above. It is not a direct statement of my opinion; it is only a hypothesis of how governments such as the United States might practically implement non-traditional forms of marriage, dictated primarily by logic. If you want to hear my personal opinion, keep reading. Though this article no longer discusses making marriage gender-blind, I am personally glad that it is so in the United States. There is no good reason to limit marriage to opposite-sex couples, and attempting to enforce such a limit can become tricky with the emerging recognition of nonbinary sexual and/or gender identities. Group marriage, if implemented fairly, I would support; I know there are people out there in ‘odd’ arrangements who would be normalized if they could be married completely, and I see no reason to deny them that normalcy. Non-exclusive marriage I can accept if implemented fairly and there is public support, though I would prefer it be called something else, for moral and practical reasons. Marriage with non-humans is fine with me, but that point is moot until some animals are legally declared ‘persons’ or aliens land; until then, you can't marry your pet because your pet can't enter into legal contracts. The section on religious freedom is just common sense as far as I'm concerned, though recent developments in this area suggest there's lots of room for debate. I'll admit I already let my opinion slip regarding commerce, but to be honest I'm not entirely sure myself precisely where the line between legal and illegal discrimination should be.