The definitive dictionary of the English language, the OED, does not contain a single instance in which "modern" civilized society has included gay marriage.  It does mention instances of "group" marriage in small, primitive societies, where all the men in a village are married to all the women.  But those, as you know, are by far the exception.  

Actually, your argument bolsters my point.  It was so universally understood at the founding of the Nation that marriage meant man-woman that marriage did not need to be defined.  Indeed, in most States, marriage could not have been defined so as to allow gay marriage, because until 1961, ALL 50 STATES outlawed sahdemy.  Do you begin to get at least part of the point?