My startup has an unofficial hiring policy based on race. Is this normal/legal?

This is going to sound like troll bait, so I'll start off with a disclaimer: I fully support organizational diversity and I think any tech company would be better off with more women and nonwhite people in its ranks. I want to have a frank discussion about an important subject, not start a political mud-slinging match, so take that elsewhere.

Here goes:

My startup of ~10 people is outright rejecting all candidates and referrals who are white males. (Yes, really). We just got our series A and have been expanding our personnel, but off the record, management and investors are concerned about the optics of having a company with only white dudes (the company is currently 100% white males). We are still a young enough company where leadership has candor with the rank-and-file engineers such that they feel comfortable being honest with us about this.

As a result, in a meeting yesterday afternoon, engineers were specifically told to stop referring candidates from our networks who are white males. They will immediately be rejected.

Has anyone else experienced this in a young company? How did it go? How common is this approach generally? Is a policy like this ethical and/or legal?

Once more, please keep it civil - I know it's a controversial subject, but I'm interested in a thoughtful discussion, not in arguing with trolls. Thanks.