I always wonder about that also. Because proposed ballot initiatives are reviewed and have to go through a process to be approved to gather signatures. Then, the signatures have to be reviewed to ensure that enough valid signatures are on the petitions. Part of the vetting process is that a proposed initiative has to be legal in the first place. For example, you couldn’t have a ballot initiative to take away women’s right to vote, because that would conflict with established federal law and the constitution.
So, clearly, a determination was made that it was legal for the voters of California to vote on the definition of marriage. The gay activists only started saying it was an illegal improper subject to vote on after Proposition 8 was approved.
Its more than that, moonbeam specifically took it to the supremes for their review to try and have it removed from the ballot and failed.