I think the argument lies in the pursuit of happiness and anyone being allowed to marry anyone they want (i.e., equal rights) so long as the other person is deemed to be able to consent. (THus leaving out age, animals, etc. There are some other stipulations as to number of people involved and familial relations. They're interesting and I'll go into them at some point if you care to find out from me.)
There is nothing in the universe which makes human beings preordained for marriage. It is a contract, a covenant if you will, which is by tradition and intent reserved as an agreement between and man and a woman.
Personally, I don't care who you live with. And any person in this country has the same rights to property, personal safety and liberty as any other.
I don't see that altering the meaning and purpose of marriage serves a greater good.