In my mind, I would say that the primary difference between the struggle for both groups is that there is a fundamental barrier to the reconfiguration of marriage. One of the dominant arguments to arise from the opposition to same gender marriages is that "marriage has been traditionally defined as being between a man and a woman." This "traditional" argument has found basis in both religion and national mythology. It seems to me that those who were advocating for interracial marriage did not have to take on this battle. Their battle was broached as an issue of Civil Rights, which concurrent with the time period's emphasis on the need to pass Civil Rights legislation, did not seem to stretch the conception of marriage. For those who wish to ensure same gendered marriage, there is a civil rights issue, but also one that seeks to redefine the basic tenets of marriage in terms of its definition and scope. I think that this is probably one of the major differences between both groups' attempts to seek out the validation of their own experience and the authentication of their own voice in a social and political setting.
This is a really tough question to talk about objectively.
I would say that the major difference between the two is that same sex marriage is seen by more people as a complete departure from one of the major purposes of marriage.
Although this is not the only purpose of marriage, one purpose of marriage is supposed to be to provide a stable home for children that come out of the marriage. Of course, we do not really follow this ideal as much as we once did. We have straight couples getting divorced all over the place and not providing stable homes.
However, gay couples can never, obviously, have children of their own. This makes it so that their marriages (if legalized) could never live up to one of the things that marriage is supposed to be about. I think that this is the main difference between the two.