Conservatives like to complain that allowing same-sex couples to get married is redefining marriage. The most common response to this is to argue that redefining marriage isn't necessarily bad, and to point out the many times marriage has been redefined in the past. However, I think the argument can be refuted on another level: Gay marriage is not a redefinition of marriage.
Or rather, the redefinition of marriage which logically leads to gay marriage already happened, and it happened a while ago. The change happened gradually, over a long time. There wasn't a single landmark court case I can point to like Loving v. Virginia.
A long time ago, a woman was considered to be basically the property of her husband. Even after that was the case, a woman was still supposed to be subservient to her husband. He had authority over her.
But over time that changed. Eventually, a marriage was no longer a relationship between owner and property, or between superior and inferior. It became a relationship between equal and equal. And once that happened, there was no longer a masculine role and a feminine role, and thus no need for a man and a woman. Just two people.