Yes, marriage is a legal concept defined by the states. There is no God-given word, even in the Bible, that defines what a true marriage is. In practical life, marriage creates special privileges for couples such as tax benefits, etc. The definition of who gets to receive such benefits in this legal construct should be decided by society.
Society should be able to legally define marriage, and that definition should be the legal definition. Society does not have to agree with that definition, but there needs to be a definition so that when people talk about marriage, we will all have a basic understanding of what that means.
America is a country of many cultures and many faiths, and the definition of marriage is different to different people. One religious tradition (Christianity) should not have hegemony over everybody else. People are going to partner up no matter what the law says, and the constitutional requires people be equally treated under the law. That means gay and straight people are equal and should be treated equally.
Marriage should be defined by the stance that our constitution takes on the issue. And as it were, our constitution matches up with the morals in the Bible and it endorses, nothing but, heterosexuality... Those who don't agree with this should take up their issues with our founding fathers.... They were petty straightforward about what is right and wrong.