The court case is going over which type of amendment prop-8 was and thus was the method used to pass it valid.
Prop 8 was introduced as an amendment: it added language to the California state constitution. Amendments require only a simple majority vote to pass.
The other type of constitutional change is a revision: striking language or significantly changing the language in the state constitution. A revision requires a 2/3 majority vote to pass.
Because the California state constitution already has an equal protection clause (the clause which the California Supreme Court used to declare gay marriage legal), the addition of Prop 8 to the constitution would seem to place it at contradiction with itself.
What the opponents are arguing is that for Prop 8 to be valid it would have to be a revision: striking the language in the equal protection clause and adding the language that the banners of gay marriage want.
Convincing 2/3 of Californians to strike the equal protection clause from their constitution is a much more daunting task than getting 50% of them to say, ick, we don't like gay marriage. Ban it please.