James Taranto argues that "contemporary feminism" has had more to do with the decline of marriage than has homosexuality.
The institution of marriage has been a casualty of contemporary feminism--specifically of the idea of sexual equality. In prefeminist times, as this column has argued, a husband and a wife each brought something distinctive to the marriage. The 50-year campaign to elevate the status, power and wealth of women relative to men has blurred these sex roles--and of course that is one of the goals of feminism, which views such roles as oppressive social constructs.
Same-sex marriage is a logical extension of the idea of sexual equality. If men and women are at the deepest level interchangeable, then there's nothing to distinguish a "husband" from a "wife" and no reason that a "marriage" has to consist of one of each rather than two of one or the other. (Traditionalists can take some comfort in the realization that this logic does not lead to such horribles as incest or polygamy.)
Same-sex marriage, then, is connected to the breakdown of ordinary marriage. But the former has not caused the latter; rather, they are both effects of the same cause.
I wouldn't use the word "elevate" as Taranto did in the first quoted paragraph. I don't think it's an elevation to make women more manly, and more than it's an elevation to make men more womanly. I do think that the prevailing view that men and women are interchangeable is wrong and harmful to families and society.