There is no federal law establishing my rights, so if a company tries to ask about my personal life (wife, kids, etc.) I politely decline to answer, or I answer vague enough that doesn't give them any useful information.
A ruling by the Supreme Court applying a federal statute is binding federal law nationally, it can be overturned either by the Supreme Court itself, or by changing the law that the Court applied, but that’s actually stronger than federal statute law that superficially seems to more directly protect a right but which does not have such a confirming opinion.
Strictly speaking, its not illegal to ask the question. However, because it is illegal to base hiring decisions on it, asking the question is legally dangerous in two ways:
(1) Asking the question in an employment interview is evidence which can tend to support that you intended to use it in a hiring decision, and
(2) The interviewee answering the question is evidence that you had the information, and thus the opportunity to use it in a hiring decision.
As a result, the usual legal advice is to strictly avoid asking the question: you can’t legally use the response, and by asking it you make yourself unnecessarily vulnerable. The idea that it is illegal to ask the question is probably a consequence of this.
They also tend to ask a lot about things that might sound innocuous, like hobbies.
If I was being stupidly honest, I could talk about my involvement in the furry community. Most furries (~80%) are LGBTQ (versus ~3% to 5% of the base population).
Instead, I make vague allusions to being a gamer. (I play video games less than anyone I know, but it's not zero, so that's still technically true.)