If polygamists were to seek marriage rights tomorrow, on what basis would you argue against it that doesn't conflict with Kennedy's logic?
If there is any limit rightly to be placed on marriage rights, Kennedy's opinion today does little to establish it.
I'd argue three tracks. First, that there is no established body of law governing plural marriages. Second, that plural marriages have a history of coercion of minors. Third, that dissolution of plural marriages offers unique problems in custody that would make Hell the lives of children so afflicted. None of these apply to gay marriage unless we artificially create a division.
Of course, I could just as easily argue the opposite. Case law will create the required legal precedent as it is needed. Cults already practice plural marriages in legalized form, via divorce and continued cohabitation, without the protections that would be afforded under legal plural marriage. Children are already affected by plural cohabitation, without the protections of established law or the security of a formalized, recognized marriage.
As I said, I really have no strong feelings either way. I could make the same argument about gay marriage personally - I have no family or really close friends who are gay - but with gay marriage I can recognize at a glance that the only distinctions from hetero marriage (from government's standpoint) are those we choose to create or those which exist only because they exist. Once one learns to look at questions with a view toward maximizing personal liberty - not why should we legalize this, but why should we not allow it - the question becomes self-evident. Thus I recognize a ban on gay marriage as an arbitrary discrimination that serves no real purpose. This makes such a ban a serious infringement of personal liberty for no good purpose and we should all fight that whether or not we are personally affected. The same arguments MAY be made for plural marriage, the situation is just not as immediately self-evident to me.
By the way, I agree that the majority opinion makes it difficult to place any bounds on marriage. I think this is a good thing. Government should always be in the position of having to come up with an extraordinarily good reason to disallow personal liberty.