The problem here is constitutionality- 'marriage' is basically a civil contract between two people in the States- by saying that one has to be a man and the other a woman, it becomes discriminatory as you can't legislate using age (except for those under 18), sex, race, etc.

I understand the moral implications, but if someone is gay, the inability to form a civil union to receive the tax advantages (and the inherent grief of being tied to someone) does indeed seem like discrimination.