A reader sums it up nice and neat on Andrew Sullivan’s Daily Dish:
The critical point that is put forward in every brief seeking Prop 8‘s invalidation is thatProp 8 stands for more than gay marriage: it stands for the premise that any minority group can suddenly be targeted by the majority for discrimination. It essentially eviscerates the Equal Protection Clause of the California Constitution. This because a) the Supreme Court has held (in Perez, some 20 years before Loving) that marriage is a fundamental right and b) in the Marriage Cases, that homosexuals are a suspect class. Unless Prop 8 somehow by implication reversed either of those, Prop 8 means that any time we wanted to be mean to some minority group, we could.
Emphasis mine. This is about civil rights and equal protection. And while the courts have been asked to look into it, you can sign the petition to repeal it.