In response to Matt’s point here, if I understand correctly most gay and lesbian rights groups (especially in recent years) haven’t opposed all litigation (Goodridge was the result of a carefully coordinated combination of seven lawsuits with support from LAMDA, for example.) Rather, they have opposed federal litigation, which given the non-existent chance of victory with the current composiiton of the federal courts makes sense. On the other hand, as a practical matter, it would in fact be difficult to file a serious lawsuit seeking your marriage rights without any support from prominent civil rights organizations. Nothing can stop you from filing, but without the resources to pursue the a good case through appeals, amicus briefs from prominent organizations and individuals to signal sympathetic judges, etc. your suit is unlikely to get anywhere. So prominent national organizations do have some (although far from total) ability to control the process. (And, of course, there’s often disagreement among organizations about the optimal strategy, which further complicates things.)
The potential tension between the immediate interests of plaintiffs and the demands for a coherent national strategy was also a major part of the LDF’s civil rights litigation (and a particularly difficult problem, since finding plaintiffs in the Jim Crow south, for obvious reasons, wasn’t easy.) Mark Tushnet’s book is very good on this subject.