Agree with nearly all said thus far. And I'd like to emphasize my own two cents...
What I believe has happened is this: State-sanctioned relationships give state-sanctioned benefits in many countries (Tax Rights, Inheritance considerations, Adoption consideration, legal next-of-kin issues, Do Not Resuscitate orders and more). This places a government in the unearned and precarious position of defining what is a 'legal relationship; what *kind* of relationship is worthy of such benefits and considerations.
The instant we did this, we opened Pandora's Box and let it all hang out.
Legally-binding relationships (in those countries that chose to do so) can be easily defined and apportioned to exclude requisites of Gender and Quantity. There's simply no reason not to allow all people the freedom to legally solidify their bonds in the eyes of their countrymen however they see fit! (this view, I might mention, is apart from age considerations. Children who've not yet matured to the point where they can even have a chance at making informed life-choices need protection).