You need a basic understanding of Constitutional law to understand the decision/opinion. (Note: this is not intended to be an insult by any means, EP is a very complicated subtopic within Constitutional law, which is a complicated area of the law on its own)
Essentially, when a law targets a certain group then Equal Protection is triggered, this requires the Courts to apply a standard/scrutiny. (See: http://en.wikipedia.org/wiki/Equal_Protection_Clause) Specifically, read up the section on "Tiered scrutiny". In this instance DOMA would require a "legitimate purpose" to target this group (like mid-level scrutiny), it is not that the SCOTUS could not find a legitimate purpose to over turn the law (as I believe you interpret), but that the law was overturned because the legislature did not have a legitimate purpose in creating the law (other than disparage and injure the targeted group, according to the majority opinion).
Doesn't this sort of judicial activism weaken the separation of powers
The idea of Judicial Activism is a red herring, when any court rules against ones own personal bias/interest. For example, Brown v. Board and Brown v. Board II are typically called judicial activism, but generally only by those who prefer Jim Crow laws. Moreover, this is precisely what the Court was meant to do, review a law and determine whether the legislature enacted a law that violates the Constitution, if this is "judicial activism" then what is SCOTUS for and how would they be a check on the legislature?
Comments
You need a basic understanding of Constitutional law to understand the decision/opinion. (Note: this is not intended to be an insult by any means, EP is a very complicated subtopic within Constitutional law, which is a complicated area of the law on its own)
Essentially, when a law targets a certain group then Equal Protection is triggered, this requires the Courts to apply a standard/scrutiny. (See: http://en.wikipedia.org/wiki/Equal_Protection_Clause) Specifically, read up the section on "Tiered scrutiny". In this instance DOMA would require a "legitimate purpose" to target this group (like mid-level scrutiny), it is not that the SCOTUS could not find a legitimate purpose to over turn the law (as I believe you interpret), but that the law was overturned because the legislature did not have a legitimate purpose in creating the law (other than disparage and injure the targeted group, according to the majority opinion).
The idea of Judicial Activism is a red herring, when any court rules against ones own personal bias/interest. For example, Brown v. Board and Brown v. Board II are typically called judicial activism, but generally only by those who prefer Jim Crow laws. Moreover, this is precisely what the Court was meant to do, review a law and determine whether the legislature enacted a law that violates the Constitution, if this is "judicial activism" then what is SCOTUS for and how would they be a check on the legislature?