Equal protection generally requires a legislature to have a rational relationship between a classification and a legitimate governmental purpose. For example, requiring drivers to be over age 16 is rational because people under 16 are more likely to get into accidents, and its purpose - to reduce car accidents - is legitimate. The classification based on age is rational even though there could be some 15 year-olds who will drive better than some 21 year-olds. If a laws classification is not rationally related to a legitimate governmental interest, it will be stuck down as unconstitutional.
Opponents claim that excluding same sex couples from marriage is rational because marriage is meant to promote child rearing. However, California currently treats civil unions and marriage the same, only applying different labels to the legal relationship. There must be some rational, legitimate reason for applying the label "marriage" to one relationship and "civil union" to another. The real reason people oppose using "marriage" for same sex couples is (1) to stigmatize homosexuals, and (2) for religious reasons: they believe "marriage" is sacred and homosexual relationships are sinful. Neither of these reasons is a legitimate governmental purpose. Thus, using the term "civil unions" does not have a rational purpose and should be held unconstitutional.
It could be argued that having a different name is rationally related to being able to differenciate for purposes of federal law, but this could be done in ways that are not designed to stigmatize.
This same logic would not apply to other States.