This Seems to Answer Our Legal Question

The petition charges that Proposition 8 is invalid because the
initiative process was improperly used in an attempt to undo the
constitution's core commitment to equality for everyone by eliminating
a fundamental right from just one group - lesbian and gay Californians.

[...]According to the California Constitution, such radical changes to
the organizing principles of state government cannot be made by simple
majority vote through the initiative process, but instead must, at a
minimum, go through the state legislature first.

http://www.americablog.com/2008/11/lawsuit-filed-against-anti-gay-prop-8.html