For me it comes down to an issue of rights. When a couple gets married they are granted certain rights, privileges, and responsibilities by the government. These rights should be granted, not denied, to gay couples as well. Whether you want to call it a marriage or a civil union or whatever, for me you are just mincing words.

Say your a man and your wife is in a horrible accident, you would be allowed to see her in the hospital. If you're gay and your partner was in an accident, you might not be allowed in to see him (or her) since you don't have any documented relationship with that person. These are just some of the rights I'm talking about, not to mention tax breaks (or burdens) etc...