Legalizing polyamory & polygamy; includes Heinlein
In the first place, moving from traditional man-woman marriage to marriage equality is very straightforward. Indeed, equal marriages are *easier* to fit into our system of laws than traditional marriage, because all you need is two consenting adults who aren't too closely related -- you don't have to legally define "man" and "woman". Once women have all the legal rights of men, equal marriage for same-sex couples was IMHO inevitable, because things that are equal to the same thing are equal to each other.
But polygamy does not map so simply onto the pattern of conventional marriage.
There are two basic categories of polygamy that people bring up when they're talking about how legalizing same-sex marriage may lead down a "slippery slope" to all kinds of kinky multiple relationships (not to mention the box turtles).
Traditional polygamy -- as found in the book of Genesis, among "fundamentalist" Mormons, in Islam, pre-modern China, etc. -- is what biologists call "polygyny", one male mated to more than one female. In most (all?) traditional societies, polygynous marriages are legally a set of overlapping monogamous marriages: the man is married to each woman separately. The co-wives do not inherit from each other, they do not get custody of each other's children, they cannot sell each other's property.
In recent decades there's been some development of the concept of polyamorous marriages: multiple-partner marriages in which all parties are considered married to each other, regardless of gender. Property is held in common, but I don't know what the usual arrangements are for child custody, powers of attorney, inheritance, and so forth, or if there *are* any consistent patterns being developed.
I know of no culture in which this kind of egalitarian polyamory is traditional. The examples that spring to mind are all in science fiction. In fact, as I sort through examples in my mind I'm coming up with more egalitarian-poly sf cultures than traditional-polygyny cultures -- can anyone think of an example of an sf or fantasy novel with traditional polygyny where it is *not* presented as something to be fled? I'm drawing a blank. Does Orson Scott Card ever show polygamy? As a Mormon, his view is liable to be more textured than most, because it's a volatile religious issue either way and because he probably saw polygamy in action while he was growing up.
I was a big fan of books about polyamory while I was young -- Heinlein's The Moon Is a Harsh Mistress and Donald Kingsbury's Courtship Rite are two examples that spring to mind -- but as I get older and more realistic (you might think "jaded" or "cynical") I see the crucial aspects of poly marriage that they don't explore.
Take "The Moon Is a Harsh Mistress", for example. The protagonist, Manuel, is part of a "line marriage": the married group adds younger spouses over the decades, alternating sexes, so that the marriage does not end.
Heinlein emphasizes the sexual & emotional benefits of this kind of marriage, but he doesn't really go into what I now see as the core issues of marriage: property and status. The great benefit of line marriage would be that the property never has to be broken up: there is no generational transfer. The marriage becomes a kind of corporation, a way to concentrate and perpetuate wealth.
In TMIAHM one of the daughters of the family is married back into the line, which Heinlein presents as both reasonable and romantic. What he doesn't present is how this makes her the only true heir to the family wealth & influence, how it cuts the other children of the family out. Normal human behavior predicts that there would be a bitter struggle among the spouses to have one's favorite child be the heir, and it could easily lead to hellish levels of incestuous pimping.
Even without that, I don't know that the poly community -- or even the religious polygynist communities -- have got a handle on the issues that are the core of marriage as a legal institution. The legal issues aren't about how people live and sleep and work together, but more about transitions: medical decision-making, inheritance, insurance payments, child support.
Our current marriage law is *barely* able to deal with the complexities that arise when a marriage has only two partners, and that despite hundreds of years of experience dealing with traditional (inegalitarian) two-partner marriages. I know of no long-running legal tradition with egalitarian poly marriages, or even inegalitarian marriages but where the wives are legally married to each other. Without this kind of experience, we don't know how these relationships would "play out" legally. I don't think we can or should have legalized polyamory until polyamorists have built up legal structures & experience with them.
I don't know how well-organized this is, and there are other thinky thots I wanted to work in, but I shall stop now because it's probably the last warm Sunday afternoon of the year and I'm going to clean up my garden OR ELSE.