I think the big thing we are discussing here is what the purpose of marriage is.  

Some of the answers I can discern from these comments, article, and blog are (in no particular order):

a) a healthy, preferred means to have and raise children.
b) an archaic religious convention
c) an option among many for a long term committed relationship.
d) a relationship built on understanding, respect, commitment
e) a civil contract
f) a religious covenant
g) the only appropriate relationship for sexual activity.
h) a commitment between two individuals who grow and change and fluxuate throughout time
i) forever. 
j) the "end goal" / final stage of commitment

To be honest - we don't have one definition of marriage in the world today.  Technically, as far as my state (Iowa) is concerned - "marriage is a civil contract
between two persons who must be (1) 18 years of age or older; (2) not already or still legally married to someone else or to each other; (3) not closely related by blood or first cousins; and (4) legally competent to enter into a civil contract." But that's not the same as the next state over, and that in no way defines what any justice of the peace or licensed/ordained clergy person may require or how they may define it. 

In other words, this is a great discussion to be having!!! =)