If those who are advancing gay marriage are to be belived, then the union of a man and woman has always been a variant of the union of persons, that biology and the possibility of reproduction were never at the core of what marriage is but additions to it, that consummation was never central to the completion of a marriage since only practical when the “union of persons” happened to be members of the opposite sex, that “man and wife” were never something that made a relationship a marriage but were always a species of the genus “union of persons.”
The only problem with construing marriage in these terms is that this has never been how it was understood, even among cultures like ancient Rome which might have been most inclined to understand marriage as the union of persons. Those who take this view are thus pushed into the corner of having to acknowledge that throughout most of human history the laws, customs, culture and language built up around marriage was based on a misunderstanding of what marriage actually was, for until recently no one understood that marriage has actually always been the union of persons.
Now let’s be clear: the fact that marriage has never been understood as a union of persons does not itself prove the new concept to be faulty. However, at a minimum it does establish that it is a new concept, a novel definition that is discontinuous with the institution of marriage as it has been understood and practice for thousands of years. This is something the champions of gay marriage are reluctant to acknowledge, since their case for “equal access” depends on maintaining some degree of continuity with the norms of an existing institution. This pretence of continuity enables them to form their arguments in quantitative terms, as if they were merely expanding the pool of people who can get legally married, rather than qualitatively altering the very essence of what marriage is.