The meaning of words change over time; I think we can all agree with that. Both cultural and technological changes serve to modify (usually expand) a word's meaning. For example, the word "urban" (and it's sister "urbanization") has undergone a major expansion in meaning over the 20th century. We have words like suburban & exurban to indicate types of "urbanization". We use words like "inner-city", "outer-city", "metro area" and so on to indicate various subsectors of an urban world. These notions were inconceivable to an 18th century citizen. Yet, here we are with a word that has expanded in meaning over time. That is, using the word "urban" has a far larger meaning now than it did 300+ years ago. What would have happened had a law passed in the 18th century concerning urban areas? Do you think we would have kept to its original meaning as the authors of the law could have known it or would we interpret the word within the context of our time? I believe the latter.
I believe marriage has also undergone and is undergoing a similar transformation from its traditional meaning. Traditionally marriage was exclusively for opposite-sexed couples of the same race and religion. Now, people of different religions and different races get married all the time. There is no leap in logic to conceive that the word will (eventually) encompass same-sexed couples as well.
