I just told you what other groups we discriminate against.
We can't define marriage as a union of one man and one woman because we have to include same-sex marriages. Well what about polygamists? What if they claim that they were born that way?
I don't see any real difference if those polygamists are all willing and of age.
my bad, i was skimming. i think the idea is that we "discriminate" against some practices because they directly (human sacrifice, female circumsion) or indirectly (polygamy- this one is a little more of a reach. i think what went on recently in texas was an egregious example of the government overstepping its bounds) violate the rights of others. to make an argument that two gays or lesbians getting married violates anyone else in any legal sense is, well, silly.
now, if you'll excuse me, it's friday and i must get my drink on...