This legislation would require a court, when determining the best interests of a child in a child custody or visitation proceeding, to consider, among other comprehensive factors, a parent's affirmation of the child's gender identity or gender expression.

Never mind all the other legislation in other states and nationally that has been used to restrict trans rights that has passed, Newsom is sure that courts will strike down all of those eventually.

A visual of his logic:

And if they don't override his veto, he's effectively correct, in California at least