A stake in the matter

NPR reported this morning that yesterday San Francisco's new mayor followed the state's constitution and allowed about 100 same-sex couples to get married. (Bravo!) What was also reported is that it was expected that "conservative groups" would file a law suit today.

As I understand it (correct me if I'm wrong) but don't you have to have a stake in the issue in order to file suit. Just exactly what is their stake? How are they harmed if same-sex marriages are allowed?

Update: Later in the day a suit seeking a TRO was filed by the Campaign for California Families and the Alliance Defense Fund.