A summary of the events:
1. DOMA is under attack from multiple lawsuits around the United States.
2. The Justice Department traditionally defends legal challenges to federal laws. (I don't understand this.) However, the Justice Department decided no longer to defend DOMA, leaving it up to the House of Representatives to hire an independent law firm to do so, which it did. (I don't understand this, either.)
3. The House of Representatives hired King & Spalding, an old and venerated law firm, to defend DOMA. Human Rights Campaign, the largest sexual minority advocacy organization in the United States, then contacted King & Spalding's other clients, informing them that King & Spalding were defending a law that denied civil rights to millions of people.
4. King & Spalding felt pressured to drop the House of Representatives as a client, but King & Spalding's lawyer decided that he was required by legal ethical rules to maintain representation, in spite of his law firm's decision to withdraw, so he stayed on as the lawyer defending DOMA, but moved to another law firm.
5. The lawyer was hailed by the media as an ethical hero, and HRC was demonized as a bully engaged in unfair play. The situation spawned hundreds of news articles and opinion pieces about the nature of lawyering, the importance of respecting the legal process, and the necessity of having lawyers defend unpopular clients.
DOMA was passed in 1996 when same sex marriage seemed imminent in Hawaii. It has three sections. Section I just says what its name is. Section II says that states aren't required to recognize same-sex marriages from other states. Section III says that the federal government does not recognize same-sex marriages.
Section II tracks the language of the Full Faith and Credit Clause of the Constitution. Most of the legal challenges seem to focus on Section III.