The concept of marriage was invented to keep track of property. Children who were to inherit their parents' property had to be "legitimate". The government needed to know who to tax. "Love" has NOTHING to do with the institution, so it shouldn't matter who marries to the government as long as they know who's property to tax. The Church backed up the government with the threat of "sin" to enforce the law of the land. Even the Bible says to render unto Caesar what belongs to Caesar = taxation.
If gay people want to inherit each other's property at death, there is nothing the government or law should do to prohibit it.
More information about formatting options