Lee does not argue the financial drain, but I will. The government registration of marriage and enforcement of its terms (such as laws against adultery or bigamy) are an obvious cost. While the state does receive fees from the issuance of marriage licenses and from performing divorces, I suspect that these don't cover the cost of trying and jailing the few whose betrayal of the marriage vows is so egregious as to require the gentle ministrations of the law.
So why not just eliminate marriage as a legal institution? Change the laws to ensure that any group in any living arrangement who choose to call themselves a "family" (a term in law) with the same total income and the same number of people pay the same taxes and receive the same assistance. Change the inheritance laws to eliminate the spousal preference - or to extend that to any person whom the deceased specified (premortem, preferably). Demand that institutions cease using the pointless - and no longer state issued - marriage license in determining default medical power of attorney, joint ownership of property, ability to live in the United States, and all other rights and benefits that currently come with a licensed marriage.
Why aren't people demanding that instead of demanding that marriage be expanded to include the arrangements that they desire?
Because it's a non-starter, that's why. There's a constituency that believes that marriage is so good, so important, so (if you'll pardon the expression) sacred that any attempt to eliminate it as a legal institution (never mind that it would still be a social convention and a religious sacrament) would be met with instant ridicule and attack. A person who spoke up this way in a public forum would be booed and shunned; a politician who attempted to enact it would be drummed out of office - and possibly excommunicated.
Why is that? I believe that would be a fascinating field of research (which I have to believe someone has already looked into). To speculate, this comes from three areas:
- Care of Children: Rearing a child, much less 20 or 30 (see Bach, Johann Sebastian), is a time consuming and expensive proposition. While it is certainly possible for one person to do this "solo" (often with hired help, assistance from family and friends, or governmental assistance), it is far easier when done by at least two adults. By placing legal roadblocks on a parent (let's face it, this usually means a guy) walking away, the state has done its part to help.
- Religion: Major religious sects recognize marriage and set rules for it. Given a choice, the adherents to those religions will demand government enforcement of those rules. While these may be watered down somewhat due to multiple religious traditions, certain basic rules (you can't leave once you're married, you can't have sex with someone not your spouse if your spouse objects, etc.) will be the law of the land.
- The power of sex: even today, marriage and sex go together. The marriage license is not only a license to have sex, it limits who a married person can legally have sex with. While one might "hike the Appalachian Trail" (if you know what I mean), if one is caught doing so one might go directly to jail, lose one's possessions to one's spouse, or both.