What's so special about the term "marriage"?

My mom and I were having a talk about gay marriage today.

We’ve both noticed that many gay people wouldn’t be satisfied with a legal union. They want the marriage label. She thinks a lot of the problems with it being legalized is the title of marriage frightening some people. You know, the whole “marriage is between a man and a woman” argument.

She asked why I think gay people (in a vast, whole-group generalization) find it important to have an institution called “marriage” if another type of union would afford them the same rights and responsibilities.

I can’t even begin to know how I would feel if I was told I could have a union but not a marriage, and I couldn’t answer her in a way I was happy with.

So, straight and gay and in between jellies, why is the label marriage important, or not important, to you?

For the record, both my mom and I are 100% pro-equality, and I think if people want it called marriage, then it should be. I just want to know why they do.