Marriage is not an arbitrary construct which can be redefined simply by those who lay claim to it. Broadening the definition of marriage to include same-sex unions would stretch it almost beyond recognition--and new attempts to expand the definition still farther would surely follow. On what principled ground can Andrew Sullivan exclude others who most desperately want what he wants--legal recognition and social acceptance?
Why on earth would Sullivan exclude from marriage a bisexual who wants to marry two other people? After all, exclusion would be a denial of that person's sexuality.
The same holds true of a father and daughter who want to marry. Or two sisters. Or men who want consensual polygamous arrangements. Sullivan may think some of these arrangements are unwise. But having employed sexual relativism in his own defense, he has effectively lost the capacity to draw any lines and make moral distinctions.
Forsaking all others is an essential component of marriage. Obviously it is not always honored in practice. But it is the ideal to which we rightly aspire, and in most marriages, the ideal is in fact the norm. Many advocates of same-sex marriage simply do not share this ideal; promiscuity among homosexual males is well known. Sullivan himself has written that gay male relationships are served by the 'openness of the contract' and that homosexuals should resist allowing their 'varied and complicated lives' to be flattened into a 'single moralistic model.' But that 'single moralistic model' has served society exceedingly well. The burden of proof ought to be on those who propose untested arrangements for our most important institution.
This is a large, tolerant, diverse country. In America people are free to do as they wish, within broad parameters. It is also a country in sore need of shoring up some of its most crucial institutions: marriage and the family, schools, neighborhoods, communities. But marriage and family are the greatest of these. That is why they are elevated and revered. We should keep them so.