"By the old laws of England, the husband was called the lord of the wife; he was literally regarded as her sovereign, inasmuch that the murder of a man by his wife was called treason (petty as distinguished from high treason), and was more cruelly avenged than was usually the case with high treason, for the penalty was burning to death....
I believe that [women's] disabilities [in the public sphere] are only clung to in order to maintain their subordination in domestic life; because the generality of the male sex cannot yet tolerate the idea of living with an equal....
Marriage is the only actual bondage known to our law. There remain no legal slaves, except the mistress of every house."
-John Stuart Mill, The Subjection of Women
The next time a "marriage defender" tells you that marriage must be between a man and a woman because men and women are inherently very, very different, I encourage you to explore that assumption with them a bit more.
Usually, this alleged gender difference implies dominance of the male variety. Even if the modern "marriage defender" isn't brave enough to say so anymore or doesn't understand the historical "traditional marriage" from which our institution has evolved, we need to continue questioning why some people insist on clinging so strongly to this "one man, one woman" relic.
It isn't so much the idea of marriage as an institution that is allegedly "devoid of gender" that some people fear same-sex marriage implies, but the idea of marriage devoid of gender hierarchy.
(And also, if you weren't familiar with this particular work of John Stuart Mill's, were you pre-attributing this quote to a raging feminazi of the Andrea Dworkin variety?)