"This bill would impose a tax on the sale of, or the storage, use, or other consumption of, tangible personal property that is adult material, as defined, in this state at a rate of 25%."
It seems to me the tax would likely be unconstitutional. Content-based taxes on the sale of First-Amendment-protected materials (and recall that the law targets not just unprotected and illegal obscenity, but also constitutionally protected pornography) are generally forbidden, see Arkansas Writers' Project v. Ragland (1987).
The backers of the law seem to rely on the Court's "erogenous zoning" cases, which allowed special zoning restrictions on sexually themed entertainment on the grounds that this entertainment causes "secondary effects," in the form of crime by patrons and a decline in neighboring property values. But this law is not a zoning restriction, and would extend to places that have no consumers at all. The Court might be willing to recognize other effects as "secondary," such as the possible harms (e.g., sexually transmitted disease risks, or even mental health risks) to performers. But the law seems to be quite ill-fitted to avoiding or remedying such harms, given that the law isn't at all calibrated to these harms: For instance, the law applies to distribution of porn created outside California, which creates problems that aren't remotely remediable through a tax paid to California authorities.
Moreover, parts of the law's stated rationale -- that pornography "[e]ncourages unsafe sex by consumers," "[o]ften encourages sexually aggressive behavior towards women," and "may negatively influence [children's] developing attitudes towards sexuality and relationships" -- focuses on what the Court has treated as primary effects (the tendency of speech to change people's attitudes, and the behavior that flows from those changes) rather than "secondary effects." See, e.g., United States v. Playboy Entertainment Group, 529 U.S. 803 (2000). I think the "primary effects" / "secondary effects" distinction isn't conceptually sound. But it's pretty clear that these effects can't count, under the Court's precedents, as secondary effects.
The strongest defense of the tax would be that pornography, including constitutionally protected pornography, is different: Though it has some constitutional value, it's of lower value, and may therefore be more broadly restricted. The Court has seemingly adopted this view in some of its "erogenous zoning" cases and broadcast regulation cases.
Nonetheless, the Court has at other times disapproved of this "low-value speech" position; and more recently (for instance, in the Playboy case I cited above) it has generally judged content-based regulations of nonobscene pornography pretty much as it has judged content-based regulations of fully protected speech. Moreover, the erogenous zoning cases stress that they involve land use restrictions, and focus on the harms caused by the physical presence of pornography consumers on the parcel of land involved.
My strong suspicion, then, is that if the law is enacted, both lower courts and -- if it comes to this -- the Supreme Court will ultimately strike this tax down just as they would other content-based taxes.