To the first bold, I agree that marriage as an institution is currently failing irrespective of the inclusion of same sex couples into the category. Many reasons, but ultimately, there isn't enough resistance (societal or legal) to prevent people from ending marriages. And in some ways, the ending of marriage is promoted and glorified.
To the second bold, Without the societal gatekeeping part, marriage as a category is essentially meaningless. Throughout ALL of human existence, there has been a specific category to distinguish the pair bonds which are capable of producing future generations of human existence in the form of family units. In terms of perpetuating the human race, these pair bonds are essential and should be protected and promoted. To protect them, there needs to be gatekeeping of the institution of marriage. Seems unfair on its face, but I don't see how the category can both keep its most essential purpose and also allow anyone and everyone into the category irrespective of their capability to exercise the primary function of the marriage.
As I see it, the problem is when the government grants special privileges based on marriage status. In that context, it is unfair to deny the privileges to same sex couples. So, in the legal context, they should be allowed into the category.