Because marriage is a legal contract like any other.
I came to the same conclusion a while back.
The government needs to be out of the marriage business - we as a people need to demand it. It's doubtful that this will happen, however, because people now look to the government for approval and protection.
Ironically, for those concerned about such things, government involvement in marriage won't prevent gay marriage - at some point it will help facilitate it.
Continued government regulation of marriage simply means that in addition to gay marriage, the government will have an excuse to further regulate our private lives.