Race is an artificial construct based on nothing but a few cosmetic physical features, such as skin color. People of different "races" can, and do, breed and procreate as a result of sexual activity.
Gender is a basic difference that goes down to the chromosomes. If two people are not of different genders (male and female), no procreation or breeding can occur. Sexual activity can occur. But not procreation.
And, again, you still haven't drawn a meaningful distinction between gay marriage and polygamy. At best, you've raised some sort of vague "it's wrong because it's wrong" argument which really just boils down to the same sort of religious argument that you readily dismiss in the case of gay marriage.
