I was reading an article recently against gay marriage by a child of a gay couple. One statement from the article really struck me:
I support same-sex civil unions and foster care, but I have always resisted the idea that government should encourage same-sex couples to imagine that their partnerships are indistinguishable from actual marriages. Such a self-definition for gays would be based on a lie, and anything based on a lie will backfire.
I think that this is the heart of where the concern from conservatives comes. There are many practical problems with gay marriage. Many social problems with gay marriage. Many moral and spiritual problems with gay marriage. However, even more than that, eating at the back of our minds, is the knowledge that, whatever you want to call it, it really isn't marriage, and playing pretend always leads to bad social consequences down the road.
For instance, when we play pretend with money, it eventually backfires. When we get into debt, and misperceive and mislabel that debt as wealth, then we run into problems. This is true for the individual going on a credit card shopping spree. It is also true of the government, providing services and being hugely in debt. Because the true nature of the money was misunderstood, many actions - some of them very subtle - compound together to lead to large societal problems down the road.
Similarly, if we pretend that sexual relationships between male/male and female/female are in the same category as marriage, we deceive ourselves again. Of course, this is based on even earlier deceptions about male and female. Our current society has lied to us to say that men and women are exactly the same except for the specific sexual organs. That is the big lie that undergirds all of the small lies, and is at the heart of why the male/female relationship is totally different than male/male or female/female.
The second lie that undergirds the lie of gay marriage is the lie that societal ideals don't matter anymore. That is what is behind all of the "what we do doesn't matter to you". Even if it were true that your actions behind closed doors doesn't matter, marriage is about defining the cultural standard. So, when you bring it out into the public definition of marriage, it actually does affect us all. The goals and virtues of a society eventually get reflected in its children, no matter whose they are. It is a simple falsehood to say that it doesn't.
Marriage is about the ideal of a united humanness. There are two sides to humanity - male and female, and marriage is the celebration and the substance of the bond that joins them together. To call a male/male or female/female relationship a marriage is to simply misunderstand the term.
This blog has touched upon several larger-scale issues with gay marriage and society, but I think even deeper, that we will lose on a thousand subtler issues the very fabric of our society, because we will constantly be repeating to ourselves lies about who we are and what makes humanity whole.