The Traditional Definition of Marriage

With the Obama administration backing away from DOMA, it makes me wonder why we allow the right to get away with the false claim that "the traditional definition of marriage is one man and one woman" as if this has been what the institution has meant from time immemorial and they can thereby make some supposedly non-fallacious appeal to tradition in "defense" of it. The fact is, the traditional definition of marriage is when a man whose property includes a female child, decides to unload this burden by bribing some other guy to take this piece of property off his hands. The traditional definition of marriage is a contract where women are treated as mere property to be exchanged from one man's ownership to another. And like other pieces of property, there's no reason why a man should be limited in how many he owns. Polygamy is perfectly well a part of the traditional notion of marriage. So, the fact is that we've redefined marriage several times already. If that's the case, then clearly we consider the notion plastic enough to be capable of occasional updates for moral purposes. If that's the case, why not one more?