Jay,
In addition, I have the following comments:
I agree that it would be best to get government out of marriage (as well as most/all? other activities). But I think you overlook some facts.
You seem to think that being (legally) single is a preferred state, with a clear advantage. But, there are both legal advantages and disadvantages of being married. There are literally thousands of ways single and married people are treated differently, not just divison of property as you point out. Some of these advantages of being married can be obtained through the use of expensive lawyers, filing wills, deeds, and etc., and others are completely out of reach, legally (e.g., advantageous tax rates).
In fact, being straight, one can use a prenuptial agreement, as you point out, to retain some of the advantages of being single, overriding community property laws, for instance. Why doesn't everyone avail themselves of this option? One reason is that it is expensive. Now, imagine being gay and having the complimentary problem; being forced to be single and trying to approximate legal marriage via dozens of such contracts and filings. So straight folks have the option of working towards their ideal contract starting from either the "legally single" or "legally married" state, but gay folks must work from the single position only.
Taking income tax as a case in point. I know of straight couples who decided to either stay single or get married to a large degree on this single issue. If both have similar incomes, being single is less expensive (in terms of taxes); with a single dominant provider, being married is advantageous. While many succumb to social pressure and pay the "marriage tax", at least some refuse. At least they have the choice.
Also, there may be advantages of being married in non-government realms. For instance, if I were legally married, my company may automatically give me a break on group health insurance for my spouse, and there would be no trouble visiting my spouse at the hospital. I may not be able to find an annuity that has survivor benefits for a legal stranger, as I easily could if we were married, and etc.
Even if my employer _wants_ to accomodate me and my partner with health insurance, they may not be able to find an insurance company offering such a plan. Nor, is my company likely to make gay "friend" benefits a significant selection criteria for picking an insurance provider. Even if a hospital _wants_ to treat me as a spouse of the patient, they may be prevented from doing so by various existing laws. Why should the government prevent the hospital from dealing with me in that way, if they so chose and both my partner and I are also willing?
So, some couples (gay or straight) may prefer the legally married option, even if you don't. Why should the government prevent two men (or women) from entering into a marriage type of civil contract, if they think it is best for them?
Another flaw I see with your example is your contention that [straight] people living together can divide property any way the see fit. In fact, they may be considered to be in a common law marriage, and the ex-girlfriend (ex-boyfriend) may still get half of the property after the breakup; either under community property laws, or as palimony. There have even been "gay palimony" suits, even though the parties couldn't possibly get married.
Rich