How does same-sex marriage affect the institution of marriage as a whole?
Same sex marriage erodes the boundaries of what is acceptable in terms of marriage.
entspeak's link to the history of gay marriage: Same-sex marriage
shows us some interesting things.
First off, in most of the cases where same sex marriage was acceptable, it was not in actuality a marriage. For instance:
In China, especially in the southern province of Fujian where male love was especially cultivated, men would marry youths in elaborate ceremonies. The marriages would last a number of years, at the end of which the elder partner would help the younger find a (female) wife and settle down to raise a family.
In Africa, among the Azande of the Congo, men would marry youths for whom they had to pay a bride-price to the father. These marriages likewise were understood to be of a temporary nature.
So the same sex marriage was a temporary arrangement. Marriage, when viewed traditionally, is not a temporary circumstance.
In the Fujian province, the "boy marriages" referred to may have been more akin to pimping your son out for cash, as bride prices were paid to marry these boys and the arrangements were temporary. Source
In addition, the Azande's had many other peculiar customs, such as incest, sex with children, and infant marriages. Source
The fact that some same sex marriages were legal does not mean they were akin to marriage.
As for the impact of gay marriage on society, we have a modern example of what acceptance of a form of marriage which is not centered around children: Scandinavia.The End of Marriage in Scandinavia
MARRIAGE IS SLOWLY DYING IN SCANDINAVIA. A majority of children in Sweden and Norway are born out of wedlock. Sixty percent of first-born children in Denmark have unmarried parents. Not coincidentally, these countries have had something close to full gay marriage for a decade or more. Same-sex marriage has locked in and reinforced an existing Scandinavian trend toward the separation of marriage and parenthood. The Nordic family pattern--including gay marriage--is spreading across Europe. And by looking closely at it we can answer the key empirical question underlying the gay marriage debate. Will same-sex marriage undermine the institution of marriage? It already has.
The family dissolution rate is different from the divorce rate. Because so many Scandinavians now rear children outside of marriage, divorce rates are unreliable measures of family weakness. Instead, we need to know the rate at which parents (married or not) split up. Precise statistics on family dissolution are unfortunately rare. Yet the studies that have been done show that throughout Scandinavia (and the West) cohabiting couples with children break up at two to three times the rate of married parents. So rising rates of cohabitation and out-of-wedlock birth stand as proxy for rising rates of family dissolution.
By that measure, Scandinavian family dissolution has only been worsening. Between 1990 and 2000, Norway's out-of-wedlock birthrate rose from 39 to 50 percent, while Sweden's rose from 47 to 55 percent. In Denmark out-of-wedlock births stayed level during the nineties (beginning at 46 percent and ending at 45 percent). But the leveling off seems to be a function of a slight increase in fertility among older couples, who marry only after multiple births (if they don't break up first). That shift masks the 25 percent increase during the nineties in cohabitation and unmarried parenthood among Danish couples (many of them young). About 60 percent of first born children in Denmark now have unmarried parents. The rise of fragile families based on cohabitation and out-of-wedlock childbearing means that during the nineties, the total rate of family dissolution in Scandinavia significantly increased.
Half of all children are now born out of wedlock in Scandinavia. By contrast, in the US, that figure is around 30-35%. This number has been relatively stable over the past 8 years.Source
Of course, the question arises on how, exactly, gay marriage leads to the increase in out of wedlock births? Why does allowing same sex couples to marry erode the meaningfulness of marriage?
To answer this, we must ask ourselves what marriage means. The answer varies from person to person (which is part of the problem). The very concept of gay marriage has moved the emphasis of marriage from children to other areas. No longer is it taken for granted that people get married so they start a family.
Before anyone jumps all over my "assumption" that children are the fundamental basis of marriage, let's examine a concept once known as the "shotgun wedding". Why would a father, whose daughter becomes pregnant out of wedlock, force the future father to marry his daughter. Of course, there are various rationales, including ensuring that someone other than himself is financially responsible for the new baby, but the key point is that having children and getting married were once considered hand-in-glove. That is, having children was once considered almost equivalent to being married.
This is no longer true. We need to ask ourselves whether or not the child raising aspect of marriage should be emphasized more, or discarded. In Scandinavia, where the importance of marriage between parents has eroded, we see the results: a majority of children are born out of wedlock. We have not seen the social implications of this, but Scandinavia certainly will over the next decade or two. I doubt anyone is encouraged by the trends there.
In order to emphasize marriage's importance in child rearing, it is important to stress the male-female bond which occurs in marriage. By de-emphasizing this aspect of the relationship we call marriage, we devalue it and therefore invite a further deterioration of the institution. We de-emphasize this by watering down the institution first off by reducing the social stigma of out-of-wedlock births and second by encouraging those who cannot, by definition, conceive children to become married.
It is not as simple as saying that allowing same sex relationships to be termed "marriages" creates a unique stress on the institution itself. However, it does add additional pressure on an institution that has been weakening over the past few decades. That weakening has caused real and measurable societal problems. Weakening it further may cause the institution to become a thing of the past, a quaint institution that used to be the framework of the family. Worse yet, it may morph into something wholly separate from child rearing, which would be a tragedy.
Edit to fix quotes.