At a Bar-b-que yesterday, I found my self talking with a family law expert. I asked him a question which has been troubling me for a while: what prevents two people who are otherwise unattached, and not closely related from marrying for tax purposes, and then divorcing. He said, nothing.
Transfers between husband and wife, or perhpas we should say between Partner A and Partner B are tax free. Hence it is possible for two businesspeople who wish to sell a business to marry, transfer cash for stock, and divorce. Voilla, a tax-free sale.
Such actions were, of course, always possible, but with the rise of gay marriage, they become much more possible, perhaps even more likely. There are many more people who are now eligible. In addition, now that the definition of marriage is now in play, the social pressure to view marriage as anything other than a status in positive law is reduced.
On what grounds would such marriages be illegal? We can't say that love is essential to marriage. In fact, marrying for money is an ancient tradition. (And how would we test it anyway?) We can't say that the desire to have children is essential, since that idea has already been rejected, at least in states where gay marriage is legal. Etc.
For the time being, the Defense of Marriage Act might mitigate the federal tax element, but I fear that law is not long for this world.