Adam Winkler, a profess or law at UCLA writes in the HuffPo:
The administration decided not to defend DOMA on the basis of a controversial reading of the Constitution. Attorney General Eric Holder's letter to John Boehner, in which the announcement was made, stated that discrimination against gays must meet what the courts call "heightened scrutiny." That means that any law singling out gays must have unusually strong justification.
If only that were the case. Twice the Supreme Court has been asked to hold that discrimination against gay people warrants heightened scrutiny. And twice the Supreme Court has rejected that argument. Instead, the Court has suggested that discrimination against gays only needed to meet a lower standard of rationality. The lower courts asked to rule on the constitutionality of DOMA so far have consistently agreed that heightened review is not appropriate.
In my view, the Supreme Court was wrong to reject heightened scrutiny for sexual orientation discrimination. Nevertheless, that's the law of the land and, for better or worse, it's the Supreme Court, not the president, who gets to make that decision.