The question of why gay couples or otherwise should have their union recognized with the rights of marriage seems to hinge on why the rights for marriage exist in the first place. So I figured this is a good starting point.

Why are unions between a man and woman allowed to legally marry, and why are they given special legal rights and recognition? How do they justify not allowing polygamists and incestial couples to marry? The question of legal rights comes down to the concept of "similarly situated" persons. Namely, if you give a right or privilege to one group, you must give it to all similarly situated people.

If marriage is just a religious union, or if it is just a definition of a union between a man and a woman, why does this therefore make it okay to establish legal rights for these unions over any other?

__________________
True knowledge exists in being aware of what you don't know.