(You might reasonably think that is just something delusional fundamentalists cry to make a slippery-slope argument, because it is something they cry to make a slippery-slope argument, but if you hang around the right parts of the world - e.g. certain corners of San Francisco or the Internet - it's not very hard to see that it's at least plausible. The interpretation of the validity or invalidity of these potential developments and society's possible reactions to it are left as an exercise to the reader, however.)
I really don't understand what's wrong with polygamy. It seems good liberal minded people should support it.
You could perhaps put some reasonable upper bound on the number of people that could enter into such a marriage, so people can't abuse it to form absurdly large tax shelters, but how can anyone justify denying 3 or 4 people the right to be married?
Personally, I have no problem with the idea of polygamy when it's, say, three people from different backgrounds who found themselves in that kind of relationship after meeting in college, or whatever.
It gets tricky when you consider the closed, fundamentalist Mormon sects and similar cult like systems, where polygamy sometimes is wrapped up with sexual abuse of children, etc.
Perhaps, if polygamy were legal, the isolated cultish groups could come out from underground, allowing them to more easily obtain wives from outside the group, which might reduce problems with the kids. But it might be too ingrained at this point.
I'm not sure how best to allow the first case, while not encouraging the persistence of the latter case.
You're asking a rhetorical question, but I'm going to answer it anyway. Please note that this post is for informative purposes, not normative ones.
The thing which is "magical", to use your term, is the family unit comprised of a mother and a father and their children. The children are exposed to both male and female influences, the combination of which is generally recognized as beneficial to their emotional development. The mother and father both have a stake in the upbringing of the children, without attendant conflicts of interest and jealousy. They may also be incentivized towards caring about future generations, building a better world for their children, and a number of other things.
Without that context of the family unit, there is, in fact, significantly less which can be said to be "magic" about the number 2. Notice that "family" occupies a significant portion of the cultural conversation of religious opposition to marriage-related issues, particularly internally-directed conversation (though the likes of 'god hates <x>' is another significant portion and usually louder / easier to find).
Again, this post is informative, not normative. Judgements on the extent to which any of these matters are desirable, and how much they ought to affect the social validity of a marriage of 3, 4, 5, or more, or marriages between 2 people (heterosexual or otherwise) who are not in the business of having children, or alternative ways to achieve any desirable goal mentioned in the above paragraphs, are once again left as exercises to the reader.
Note also that a pragmatic argument against 3+-person marriages exists, and relates to the fact that these are almost always polygynyous marriages and there may be negative social implications to having many unmarried males around, rendering societies more prone to crime, war, and other things. Again, the extent to which this is true and ought to influence social policy? Figure it out yourself.
I feel the same way. At least their are some practical arguments against polygamy. Still if everyone involved is an adult making their own decision then the government should have almost no control over who marries who.
The general point I was trying to make was that something will succeed this as the next big fight for rights and that it would be related to marriage, so people who oppose these unions should just take the smart exit and stop fighting it.
(You might reasonably think that is just something delusional fundamentalists cry to make a slippery-slope argument, because it is something they cry to make a slippery-slope argument, but if you hang around the right parts of the world - e.g. certain corners of San Francisco or the Internet - it's not very hard to see that it's at least plausible. The interpretation of the validity or invalidity of these potential developments and society's possible reactions to it are left as an exercise to the reader, however.)