That's an "equal protection" argument, and while it may be useful in establishing the degree of change to the California Constitution it's not the legal argument to be made here itself.
"Equal Protection" would be the grounds under which an argument to strike down the amendment because it violated the 14th Amendment to the US Constitution would likely be argued at the US Supreme Court, but nobody wants to take it there yet. The nice thing is that the existence of civil unions create a "separate but equal" structure, which plays nicely with the precedent created by "Brown v. Board of Education." The ugly thing is Justices Thomas, Scalia and to a only slightly lesser extent Roberts and Alito.
Re: More analysis of the legal position
"Equal Protection" would be the grounds under which an argument to strike down the amendment because it violated the 14th Amendment to the US Constitution would likely be argued at the US Supreme Court, but nobody wants to take it there yet. The nice thing is that the existence of civil unions create a "separate but equal" structure, which plays nicely with the precedent created by "Brown v. Board of Education." The ugly thing is Justices Thomas, Scalia and to a only slightly lesser extent Roberts and Alito.