But there are more obtuse Americans than there are gay Americans, so courts are going to be bulging with legal disputes among the unalert, who neglected to plan in advance and make private contracts resolving the many legal issues that are normally determined by a marriage contract.
Ann is right. The reason government licenses marriage is to streamline that common and contract law you refer to so that the courts aren't clogged up with "obtuse" Americans. It is a cheap and efficient legal convenience.
Not licensing it doesn't actually get the government out of marriage. You're just shifting the legal activity to the courts (and feeding the lawyers).
I assume the solution is some sort of state issued legal unions for hetero and homosexuals. If there has to be change, this would be the best route, IMO
Its also why gay activists don't want government out of the marriage business and don't just do a basic "sign a will/and give the partner power of attorney".
Some libertarians have proposed only allowed civil unions (recognized by the government) for straight and gay folks, while leaving "marriage" itself to whatever religious institution there is.
Gay activists are opposed to that too, they want the benefits of marriage and also the laziness of avoiding the issues of contract law and courts.