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.