What is marriage anyway?

So many people are so vitriolic about who does and doesn't have the right to get married. What is a "marriage" anyway?

Marriage really has two definitions. One civil and one religious. And they are not the same thing.

Let's talk first about the civil definition of "marriage. I'm not a lawyer, but I've discussed this with several. It is simply a contract between two people, which entitles both parties certain legal protections and rights under the law. It is very similar to an LLP or LLC in corporate parlance. The state certainly has an established right and purpose in setting certain exclusions to this sort of contract. For instance, there's no doubt but that there is plenty of precedent that the government can set a minimum age limit for engaging in such a contract. No reasonable person can deny that. The government also has an interest in the mental capacity of those who enter into such a contract, to ensure one is not taking unfair advantage of the other. LLP's, LLC's and other contractual entities also establish that there is a legal precedent for determining the number of persons who may be involved in the contract. So an appropriate number of persons of legal majority, and of sound mind may enter into the contract. So far, so good.

Name one other similar contract for which there is an established precedent in which the government has any interest, whatsoever in the gender of the parties to the contract. To my knowledge, there isn't one.

Then there is the religious definition of marriage. Currently there are over a dozen established, recognized entities, including several mainstream Christian churches, who perform same-sex marriages within their interpretations of the scriptures. The "free expression" clause of the first amendment prevents any laws which discriminate against these churches and other religious entities from performing such unions, and those unions being legally recognized within the law the same as any other.

So, please, someone explain to me how same-sex marriages are any of the government's business, and why any laws against such unions pass any level of constitutional muster.
DD