Marriage is a civil institution that relates primarily to property. The US gov't has only been involved in marriage for a few hundred years because the US gov't has only existed for a few hundred years.
Yes churches and religious groups kept marriage records and performed ceremonies. "The state" has evolved over time and for quite a bit of it religious groups were part of the state. That had some flaws - go read some European history.
Marriage as a civil institution serves some very concrete situations. It handles various situations that arise for couples in terms of property rights, inheritance rights, child custody, immigration, criminal law, taxes, pensions, etc. There are problems that crop up for couples that are more complicated than if they came up (if they could come up) for individuals.
Laws relating to marriage are designed to address those situations. To provide couples with reasonable expectations that they can plan on and to simplify people's lives in terms of their interactions with the state.
The laws relating to marriage are generally there to make people's lives run more smoothly and therefore often get overlooked. But at times they crop up and when couples (both or individually) avail of them they understand the benefit those laws provide.
Gay couples can already get married in religious services in all 50 states. The debate about the religious institution of marriage is a debate each religious group can work through. Eventually all of them will recognise same sex marriage but that's up to their individual congregations so generally the point is moot for most of us.
This civil institution of marriage (it's like Java - it's a VM, it's a language, it's a beverage, it's an island!) is the issue that all citizens should be concerned about and it should respect the rights of each citizen and weigh the various concerns. You being "offended" is way lower on the scale than gay citizens having legal protections for their relationships just as straight citizens do.
This is plainly clear. And in the future - the not very distant future I might add - those who don't understand that will be viewed by decent society as hateful, anti-social bigots. Everyone's free to be a hateful, anti-social bigot, but the rest of us are also free to call that out and to shun such people. Everyone has a right to their opinion and to speak their mind. No one has a right to avoid the social consequences of doing so.