Exactly. I tend to lean towards legalisation of many things for the same reasons - if they're out in the open, there's more chance of people seeking help with problems, and more ability for the whole thing to be policed. I usually feel the same way about prostitution, and most drugs.

I've had the argument about complexities of ecconomics and property law put to me before as a reason for not legalising group marriage. It would certainly be more complex in the instance of divorce and such like than two person marriage. At the same time though, we have a legal system that somehow deals with corporations, so I'm sure we could find a way! And on top of that, these relationships (as you say) allready exist in a de facto form. Currently the people involved have no rights to property and so on in these situations.

It's a complex issue - on the one side you have the whole alternative lifestyle polyamory people, who are usually all about communication and equality and all that nice stuff. On the other you have the full on religious groups who are generally into the one-sided man with many wives model (polygamy as opposed to polyandry or anything else) - which has a history of scary abuse of often very young women. In practice I think we usually know what we're looking at, but I suspect it's harder to separate the two legally.