Should marriage in any form have special status?

The family diversity or family equality argument is perfectly expressed today in The Irish Times by Karen Kiernan of One Family.

The essence of her argument; the family in Ireland is changing, all families are equal in esteem and marriage shouldn’t be held in higher esteem. Therefore change the Constitution and remove the special status of marriage. It is discriminatory.

Needless to say, Karen is in favour of same-sex marriage. It’s just that she doesn’t think any kind of marriage should get special treatment.

What do same-sex marriage advocates have to say in response? Do they agree with her that marriage shouldn’t have special status, in whatever form, or do they think it should and therefore they disagree with her?

If they do disagree with her, why do they disagree with her? It would be good to know.