Expanding the legal framework of marriage - why should we draw the line where we do?

http://news.bbc.co.uk/1/hi/magazine/8549429.stm
http://www.nytimes.com/2010/03/04/us/04marriage.html

Why do we think that the state has the right to intervene in some aspects of relationship (such as the expanded definition of marriage to include homosexuals) but not others. Why stop there? Why not expand the definition of marriage to allow polygamous groups of men and women to all be married if they so desire?

Similarly why do we allow certain “religious” rights to groups such as Christians, Moslems and Jews, but bar similar rights from groups of people such as Jedi Nights or the Church of the Flying Spaghetti monster, or followers of “The Dude”.

Both of these questions point in the direction of the historicist nature of our moral framework. The answer to both these questions is so obvious and yet terribly confronting to our law makers. There is no real ground upon which our moral framework rests, other than the weight of the past, combined with the relative size, determination and assertiveness of any such special interest groups.

The reason we allow air stewards to wear crosses, but not carry plastic toy light-sabres is because people have been wearing crosses for 2000 years, whereas they have only been wearing plastic toy light-sabres for 20.

And same sex marriages between couples is a more similar structure of commitment between people to the previously existing framework of marriage, than is marriage amongst mixed or same same-sex groups of an arbitrary size or self-determined composition.

What makes us think that the laws we make in such cases are right and acceptable, is not ever based on any reference to some Platonic notion of “The Good”. Rather it is simply an extension of our prejudices from our past as they colour the determination of those alive in the present to stand up for their own vision of what is personally desirable.