Buzzfeed seems dubious that the President's choice to unilaterally cease enforcing key aspects of the Defense of Marriage Act constitutes an attack -- Republican lawmakers obviously disagree:

The House Republican leadership Tuesday filed a brief in the Supreme Court urging the Supreme Court to uphold the Defense of Marriage Act as constitutional, arguing that the Obama administration "abdicated its duty to defend DOMA's constitutionality" in February 2011 and instead started "attacking" the law in court.

As to the law itself, the House Bipartisan Legal Advisory Group — controlled 3-2 by Republicans in light of their House majority — argued that the federal government had the authority to legislate in an attempt to ensure "national uniformity" regarding the provision of federal benefits. The House leaders argue that in addition to the federal reasons, the Congress could act for the same reasons many states have acted to ban same-sex couples from marrying. They wrote:

There is a unique relationship between marriage and procreation that stems from marriage's origins as a means to address the tendency of opposite-sex relationships to produce unintended and unplanned offspring. There is nothing irrational about declining to extend marriage to same-sex relationships that, whatever their other similarities to opposite-sex relationships, simply do not share that same tendency. Congress likewise could rationally decide to foster relationships in which children are raised by both of their biological parents.