William C. Duncan in National Review Online:

"...A very important part of the brief [filed by the 10 senators in defense of DOMA] is its response to “a fundamental misunderstanding of the judicial role in passing on the constitutionality of a federal statute.” The brief explains that nothing in constitutional law “authorizes a court to strike down an otherwise constitutional law based on the belief that legislators individually, or the Congress as a whole, were motivated by ‘animus.’ Adopting any such doctrine would be highly dangerous to the separation-of-powers and the proper functioning of our constitutional system.”

In fact, they explain, the accusation of animus “is little more than an attempt to win an argument by disparaging the motives of the other side.” The senators characterize what the trial court did in this case: “The District Court’s approach cherry-picks statements made by individual members, construes them as reflective of improper motivation, and then imputes these improper motives to the Congress as a whole.” Among other problems with this approach, the brief points out: “Scouring the congressional record for ‘sound-bites’ to divine and disparage the motives of individual legislators also chills the freedom of legislative speech that is the hallmark of robust democratic debate.”