A new topic at Discussing Marriage explores the argument from interracial marriage. I would summarize the argument like this:
P1: Interracial Marriage was illegal in many states previously
P2: Laws against Interracial Marriage were struck down appropriately
P3: Bans against Same-Sex Marriage are similar to bans against Interracial Marriage
C1: Same-Sex Marraige should be legalized
The response is that gender is an essential element in a conjugal view of marriage while race is not. Thus, in this case, gender and race are of entirely different categories when it comes to marriage and marriage law. To view additional explanation and references I would recommend the link above.
Eric Nielson
Small and Simple 
But you miss two things:
1) The opprobrium against interracial marriage was dead wrong, immoral, and indefensible. Even the most strident opponents of gay marriage will say, “Yeah, we were kinda wrong about that interracial marriage thing back then, but we promise we’re right now!” In the LDS church we just kind of manage not to quote certain off-putting talks from that era, in part because …
2) … the doomsday predictions of those who opposed interracial marriage have not come true. Racial harmony is still a work in progress in many countries, including the US, but the sky has not fallen and increased visibility of racial minorities (including mixed-race families) has only been a good thing. Moreover, if the government never forced any LDS temple to conduct an interracial marriage before 1978, it is unlikely to occur with gay marriage now.
Might it be possible that our fear of interracial marriage was misplaced and incorrect, and that the same might be true of our fear of gay marriage?
Thanks for your comment. I would simply restate the response, Gender is an essential part of marriage (in a conjugal view) while race is not essential in the least. This does address both items above. One may disagree with the crucial distinction between the conjugal and revisionist view of marriage, but that is a different argument.
Since gender is such an essential view to marriage, if this is overturned it could have a greater effect on marriage practices than interracial changes did. So to summarize yet again, gender is essential and fundamental to the conjugal view of marriage, while race is not.
So are you conceding the point that LDS opposition to interracial marriage was wrong? If so, while there are other arguments against same sex marriage, it becomes difficult to argue from a position of authority when past arguments were based on incorrect or immoral assumptions.
Past opinions on race within the church were really never core teachings. While eternal marriage has been a core teaching. So I will simply state again that we are talking about very different categories. This is the response, whether or not it persuades you.
There is a response to just this argument from an LDS perspective here:
http://discussingmarriage.org/the-objection-from-prophetic-fallibility/
I will try to summarize the this soon. While my recent posts probably seem shallow, I am trying to break the arguments down to their simple core. This takes a little more time then one might think. At least for me.
One way this simplistic equivalence breaks down is that the first American law prohibiting interracial marriages was passed in the Maryland colony in 1691. The experience of the Spanish and French colonies showed that with a racially mixed population, racially mixed marriages and mulatto or mestizo children are the norm. Anything else required explicit prohibition, and in order to perpetuate a black slave population such prohibition was made. In contrast, the first law specifying that marriage partners are not of the same sex was passed in 1973, also in Maryland. Imagine a way, way before-his-time lobbyist trying to pass such a law in 1873. No one would understand what he was trying to say. “You want a law that says that when a man marries, he has to be a man and his wife has to be a woman? Look, I don’t like hen-pecked wimps and domineering harpies either, but we can’t just throw them in jail.”