The Ambiguity of "Equal Rights"
An opponent could respond, with equal logic, that it is consistent with equal rights. Both homosexuals and heterosexuals have the right to marry a partner of the opposite sex, neither has the right to marry a partner of the same sex. Seen from this standpoint, the difference is not in what rights different people have but in what rights matter to different people. Current California law provides both homosexuals and heterosexuals with the marital right that heterosexuals value and provides neither with the marital right that homosexuals value.
For those readers who see this as merely a rhetorical quibble, I put the following question: Is a law forbidding discrimination against gays in housing or employment also a violation of equal rights? Seen from one standpoint, it provides the right to hire or rent or sell to the person of one's choice to those people who are not prejudiced against gays but not to those who are—a violation of equal rights. Seen from the other, it provides both groups the right to decide who to deal with on grounds other than sexual preference and provides neither the right to make the decision on grounds of sexual preference. It's just that the right it denies is valuable to one group and worthless to the other. The logic is exactly the same as in the case of California marriage law.
All of which suggests to me that that the principle of equal rights is a great deal less clear than it may at first seem.