What Iloveflowers said! Marriage is also defined by law to set out the rights and responsibilities of each partner. Remember what life was like for gay couples before they could legally marry? They weren't allowed to see their partner in the hospital. They weren't the legal parents of their own children. Their partners did not automatically inherit upon death. I could go on and on. Legally defining marriage affects every aspect of our lives.

And then there is the tax thing. The tax laws are written specifically to favor married couples. Basically, single people are subsidizing married couples. It's really shitty.