your question is a valid one............very, very valid..........and unfortunately I'm going to disappoint you, because I don't have all the answers and therefore won't claim to
this is discussed constantly in the poly community and most of us agree, gay marriage will NOT create a slippery slope into poly marriage.......at least not the quickie slippery slope some fear. the *issues* aren't these same at all, at least not in terms of practicalities............but many of the concepts used to argue for gay marriage rights really can and should (at least imho) be used to argue for poly marriage as well.
in practical terms, the most straightforward way to level the playing field would be to get rid of marriage rights for everyone............hetero, homo, mony and poly alike. I'm not sure that could or would ever happen, so aside from that, I'm the first to agree poly marriage would/will be a complicated nut to crack.
using my own life as an example (just one example, one configuration, one mindset) I can say marriage would be a very good thing for us. we have all paid into social security for years, but our spouses will never receive any benefits. we all have separate wills, leaving everything to each other, but we have no guarantee these will be honored at the time of our death. we have POA's made up, giving each other the ability to make medical decisions on each others behalf, but in reality we don't know if these would stand up to an extended family member butting in. two of our children's birth certificates have a non-bio father listed (too complicated to bore you with in this post) which gives him rights the bio father doesn't have. the bio father of those two children has to carry a POA to get medical help for all of our children, including the two that are legitimately his (they are all *his*, but you know what I mean)...........this POA has yet to stand up during an emergency room visit. each time they have withheld treatment until one of the other two of us could be contacted by phone. I could go on and on about the difficulties living in a non-legalized marriage causes, but you know them yourself so I'm not going to bother.
for us a legal 3-way marriage would make loads of sense. we've been together for 21 years, so it's not like we're some fly by night arrangement. we are well educated, financially stable and play active roles in our community. we are by no means hippies and haven't chosen this lifestyle based on religion. if I could marry both my husbands (it's the most appropriate word for us, so I do use it) we could enjoy the same protective umbrella most families live under. instead, we spend loads of time trying to plug all potential legal holes and glancing over our shoulder. it's a bit insane when you consider we've already long since been together longer than the average mono marriage in our society.
most of the poly persons we know live in triads such as ours...........either mfm triads, or fmf triads. we do know one quad (we met them at a poly conference) and through the web we've met several V's (3 persons but only 1 is relating to the other 2) and one long term W (same idea as a V, but 5 people are involved). none of these relationships are new or short term or anything less than very marriage-like.........and none are hippy sexual free-for-alls. these are all serious, family minded adults who are only looking for the same legal protections offered other loving adults in our society.
how should it work exactly? I'm not sure. I do know of poly families who have incorporated, but that only covers the financial interests of persons involved, not custodial type stuff. I would most definitely agree with whoever said the governmental type financial perks should just be split up between those who've married into the relationship..........but some things (mostly things to do with divorces) get a little trickier to imagine. ultimately the children found in these situations would need to be closely protected from the strong lean towards bio-ownership we often see in our courts. I can tell you from personal experience, my children DO have two fathers. there is absolutely no his and mine plus his and mine going on over here.........no step parenting or anything of the kind. these children would be soooooooooooo distraught if their access to one parent was eliminated via the divorce courts carrying a bias towards biology.
I do agree with you it's complicated. I won't see we see eye-to-eye because I have lived within the poly community for too long to view them all through hippy type stereotypes. from what I've seen the level of education is actually quite high, as is the commitment to loving family relationships. gay marriage and poly marriage aren't at all equal in terms of legal complications, but the same reasoning for wanting marriage rights is alive and well within the poly community...........mostly because of the same vulnerable feeling you voiced yourself earlier in the thread.
so take marriage rights/perks/responsibilities/protections away from everyone or yes, the poly world will continue to see legal recognition for the relationships we are already living within. the process will be much slower and much more complicated than with gay marriage, at least imho, but it's only natural we'd want to live on an equal playing field with other loving, committed adults in our society.