Here was the gravaman/issue to decide, from the text:
At the same time, as numerous decisions of this court have explained,
although the initiative process may be used to propose and adopt amendments to the California Constitution, under its governing provisions that process may not beused to revise the state Constitution. (See, e.g., McFadden v. Jordan (1948) 32 Cal.2d 330; Amador Valley Joint Union High Sch. Dist. v. State Bd. of Equalization (1978) 22 Cal.3d 208; Raven v. Deukmejian (1990) 52 Cal.3d 336.)
Petitioners’ principal argument rests on the claim that Proposition 8 should be viewed as a constitutional revision rather than as a constitutional amendment, and that this change in the state Constitution therefore could not lawfully be adopted through the initiative process.
As we know, the legislature outlawed same sex marriages, the CA SC ruled that violated the CA Constitution, the voters put it on the ballot and the complaint made it's way to the courts and in effect it overturned the other CA SC ruling, now, here, they had to rule if it was proper.
There was NO federal argument/element because it was a state issue.
Aside from that, the US SC has already ruled same sex marriage prohibitions do not violate the federal constitution, way back in 1971. They may entertain it again, as the ruling was a "Summary opinion".

