The most effective of all morality-based arguments for same-sex marriage, the one that persuades more people than any other argument, is the one that equates opposition to same-sex marriage with the old opposition to interracial marriage.
The argument, repeated so often that it sounds incontestable, is this: Just as parts of American society once had immoral laws that forbade whites and blacks from marrying, so, today, society continues to have immoral laws forbidding men from marrying men and women from marrying women. And just as decent people overthrew the former, decent people must overthrow the latter.
Thanks in large part to widespread higher education -- the higher the educational level, the more one is likely to hold this view -- vast numbers of Americans believe in this equation of sex (gender) and race.
But the equation is false.
First, there is no comparison between sex and race.
There are enormous differences between men and women, but there are no differences between people of different races. Men and women are inherently different, but blacks and whites (and yellows and browns) are inherently the same. Therefore, any imposed separation by race can never be moral or even rational; on the other hand, separation by sex can be both morally desirable and rational. Separate bathrooms for men and women is moral and rational; separate bathrooms for blacks and whites is not.
The second reason the parallel between opposing same-sex marriage and opposing interracial marriage is invalid is that opposition to marriage between races is a moral aberration while opposition to marrying a person of the same sex is the moral norm. In other words, none of the moral bases of American society, whether religious or secular, opposed interracial marriage -- not Judaism, not Christianity, not Judeo-Christian values, not deism, not humanism, not the Enlightenment. Yes, there were religious and secular individuals who opposed interracial marriage, but by opposing interracial marriage, they were advocating something against all Judeo-Christian and secular norms, all of which saw nothing wrong in members of different races intermarrying (members of different religions was a different matter).
On the other hand, no religious or secular moral system ever advocated same-sex marriage. Whereas advocating interracial marriage was advocating something approved of by every religious and secular moral tradition of America and the West, advocating same-sex marriage does the very opposite -- it advocates something that defies every religious and secular moral tradition. Those who advocate redefining marriage are saying that every religious and secular tradition is immoral. They have no problem doing this because they believe they are wiser and finer people than all the greatest Jewish, Christian and humanist thinkers who ever lived.
But as objectionable as hubris is, false comparisons are worse. And there is no comparison between different races and the different genders. There are no inherent racial differences; there are significant differences between the sexes. To the extent that racial groups are different, they are only because their cultures differ. But a black man's nature is not different from that of a white man, an Asian man, an Hispanic man.
The same is not true of sex differences. Males and females are inherently different from one another. We now know that even their brains differ. And those differences are significant. Thus, to oppose interracial marriage is indeed to engage in bigotry, but to oppose same-sex marriage is not. It simply shares the wisdom of every moral system that preceded us -- society is predicated on men and women bonding with one another in a unique way called "marriage."
Comparing the prohibition of same-sex marriage to prohibiting interracial marriage is ultimately a way of declaring the moral superiority of proponents of same-sex marriage to proponents of keeping marriage defined as man-woman. And it is a way of avoiding hard issues such as whether we really want all children to grow up thinking it doesn't matter if they marry a boy or a girl and whether we really want to abolish forever the ideal of husband-wife based family.
Those who wish to redefine marriage for the first time in Jewish, Christian or secular humanist history may offer any honest arguments they wish. Comparing the prohibition of same-sex marriage to prohibiting interracial marriage is not one of them.