http://en.wikipedia.org/wiki/Defense_of_Marriage_Act: "Under the law, no U.S. state or political subdivision is required to recognize a same-sex marriage from another state." (emphasis added)
That clause is exactly a forswearing of power not an exercise of power.
"Section 3 of DOMA codifies the non-recognition of same-sex marriages for all federal purposes, including insurance benefits for government employees, Social Security survivors' benefits, immigration, and the filing of joint tax returns."
You can argue that the federal government shouldn't provide any employee insurance or Social Security survivors' benefits, or regulate immigration, or collect income taxes - but if you grant that any of those are as a whole within their powers, you can't then argue that setting specific policies in those areas is outside their powers.
A federal law is not an exercise of power? It may reject applying one states law to another but that in and of itself is exercising federal powers.
Now, Why did that have to get passed in the first place? Ostensibly, under this whole 50 different nation states and a small constitutional federal govt, that wasnt needed in the first place. Dont agree? Why do we have state constitutions and govts then?