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.

Llye Denniston with SCOTUSblog notes that the new brief filed by GOP lawmakers points out how gay and lesbians can hardly be considered politically powerless:

Arguing that the federal government has the same power as state governments do to define marriage, the Republican members of the House of Representatives’ leadership told the Supreme Court on Tuesday that the 1996 Defense of Marriage Act does not attempt to exclude anyone from government benefits but seeks only to define what marriage means under federal laws and programs.   It means, as the Act says, that marriage for all federal purposes is a union between one man and one woman.

... The brief made a strenuous argument against raising the constitutional standard for judging laws that treat gays and lesbians less favorably.  The GOP brief contended that those individuals do not need such protection.  “Gays and lesbians are one of the most influential, best-connected, best-funded, and best-organized groups in modern politics, and have attained more legislative victories, political power, and popular favor in less time than virtually any other group in American history.   Characterizing such a group as politically powerless would be wholly inconsistent with this Court’s admonition that a class should not be regarded as suspect when the group has some ‘ability to attract the attention of lawmakers.’”

Both President Obama and Attorney General Eric Holder, the brief noted, have decided to “stop defending and start attacking DOMA itself,” and those developments show “the remarkable political clout of the same-sex marriage movement.”