The "full faith and credit" clause of the U.S. Constitution might theoretically cover same sex marriage, but it hasn't yet been adjudicated to such a level as to set precedent. Many of the states having their own legal descriptions of marriage as between one man and one woman also provide that same sex marriages performed in another state are not recognized. In one case, the state of Oklahoma was required by the court to recognize an adoption done by a same sex couple in another state. Although the full faith and credit clause lists no exceptions, states have ignored it (and been upheld in the courts) to the extent that almost every instance of its application is subject to adjudication.

It's certainly an evolving issue in the U.S., and I'd expect to see suits forthcoming although I have no confidence, with the makeup of our present Supreme Court, that gay marriage proponents would be successful.

As for sales tax in Michigan, one can make a gift to a family member without sales tax being assessed. This would include one's spouse. But, since Michigan passed a constitutional amendment some years ago to prohibit same sex marriages and the granting of any benefits normally given to married couples, you can be sure the tax isn't going to be waived for a same sex couple. And, I must admit to a technical error. Although our friend referred to it as the sales tax, it's actually the real estate transfer tax that she'd have to pay. This is 1% rather than 6%, still a significant amount. But, again, this tax is waived on transfers to a spouse or immediate family when no cash sale is involved.