I was watching a discussion among atheists in a chat room, and one of them said, "Isn't God's definition of marriage only his opinion?" I thought the question was both intriguing and sad. It's intriguing because the atheist assumed that God existed (which is against his own position) and it's sad because the underlying premise is that God's opinion really doesn't matter because it's only his opinion. Of course, I found that to be ridiculous.
Atheists often jump in and out of the Christian worldview in order to argue it. Sometimes they will speak as though God exists and then revert back to atheism as they attack the concept of God. Such is the case with the question "Isn't God's definition of marriage only his opinion?" So, I find this inconsistency interesting. Of course, atheists deny that God exists and regularly attacked the Christian concept of God. They don't have to assume his validity in order to do that. But this particular question doesn't do that.
Also, think about it, would anyone say that his or her opinion is as equally valid as that of God? Is the creator of the universe who knows all things and who reveals morality out of his character, not qualified to tell us what marriage ought to be? If an atheist disagrees that marriage is to be between a man and a woman, then on what basis does the atheist assert that his opinion is morally correct? Does he appeal to his own subjective experience and desires? Does he appeal to the so-called norms of society that change? Whatever the case, his view of what marriage ought to be is nothing more than opinion based on limited knowledge. But, God, according to Scripture, has all knowledge (1 John 3:20) and he is our Creator. By definition, His opinion is the one that ought to be heeded.
One more thing, when God informs us about moral standards, he is not giving us his opinion as though our idea of what is right and wrong is as good as his and he has his opinion that we have hours. Since God is absolute and all-knowing, not to mention being our judge, the revelations he gives us, share the characteristics of absolute moral truth. When he tells us that marriage is to be between a man and a woman, he is informing us about what is actually right, not just a whimsical, arbitrary declaration.