[dragonwriter]

If the definition of marriage had never changed from biblical times, polygamy would still be legal.

If the definition of marriage had never changed from the 19th century, married women owning separate property wouldn't be legal.

If the definition of marriage hadn't changed from what is one in any particular time and place in the past leading up to modern marriage laws, either much broader or much narrower restrictions on consanguinity would exist.

Marriage laws change all the time, and understanding of the relationships underlying marriage on which those laws are based also change all the time. Sure, same-sex marriage is a change -- but lots of other things that we consider fairly firmly entrenched in our idea of marriages were themselves changes, some fairly recent, to pre-existing models of marriage. The whole idea that there is some single "traditional" model of marriage that has been static for an extended period of time and which is now threatened by same-sex marriage is ludicrous.