Well, lets look at this for a sec. First off. Right now it is illegal in every state for polygamy, incest, underage, and same sex marriages to occur. Additionally, almsot every state places a set of further restrictions on marriage (like 72 hour waiting periods, blood tests, STD tests, etc.) You just don’t hear people complaining about those restrictions.
If this issue were just about two people then I would not really have a problem with it either. IN fact, I am very much for further clarifications of legal unions to allow for issues like inheritance, medical care, adoption, breakups, and a host of other issues that have come about over the few thousand years that marriage law has been developing.
The problem is that this isn’t just about what two people do in the privacy of their own home. It is how the gov’t recognizes that relationship. For instance, a spouse is the official next of kin of a person, and thus legally inherits property/debts of a person if there is no will. Couple that with the common law requirements (6 months cohabitation) and the idea that the number, sex, age, relationship of partners no longer matters and you get into a wierd situation where your roommate would inherit your property because you happened to live in the same apt for a year. There are other odd issues that crop up that also need to be addressed because abandoning the underpinning assumption of a marriage (what it is an what it is for) throws thousands of years of legal predecents out the window.
While it might be nice to think that morality should only be dictated by personal beleif, that is both unworkable and impossible. IN the end civilization is based upon the idea that that culture as a whole decides upon certain standards and then imposes that loss of innate freedom on the individuals in that culture. Individuals then contact voluntarily with the culture and agree to abide by the loss of individual freedoms for the benefit of the common good. Thus when you look at all laws, their goal is for the smooth operation of the culture, and almost every one is based upon morality. Murder and theft are the two easiest examples of activities that are immoral and illegal.
Lastly, two arguments on your second to last comment. One is, there is no right being taken away. Secondly, gov’t’s purpose is to secure the public good and ensure the smooth running of society. Thus, gov’t is all about letting idividuals know what rights they must sacrifice to operate in said society. ONe of the best examples of this is taxation. I know to be a member of society, I must pay taxes, even if I disgree with how they are spent.