This was a jaw-dropping exit, wasn't it?
Friends claim that there was some foreshadowing in an episode a few months prior to Serena's departure, but I certainly didn't suspect anything (and I think everyone's queer!). In that episode, Jack took on a sneaky businessman who had something to do with the murder of a politician's wife. Another man, a crucial prosecution witness, refused to take the stand, claiming that he was entitled to marital privilege because he had married the businessman in good old bizarro L&O NY where gay marriage was provisionally legal. Jack went to the Court of Appeals to invalidate all of the state's gay marriages so that he could force the guy to testify. Serena strongly objected to this strategy. Given her robotic style, this seemed to me to be merely the appropriate abstract, pious position for her abstract, pious character.