If you refuse to answer your employer must by law give their best guess.

Why? Who cares about this and what's the reason they must care?? Affirmative action? Race "quotas"?? How do you know it will not be used in the exact opposite manner? It's simply better to treat everyone equally (I know, radical notion) and stop this madness.

What's better is largely unknowable if you're not collecting statistics to measure the impact of policy changes.

Really? That’s hilarious!

In theory, yes. In practice Is it really though?

I have declined at least once, never heard anything about it