That question is illegal in the US. There is no way to honestly answer it without revealing information that they cannot legally ask, and thus they cannot legally ask it. You should stop the interview right there, if they are willing to violate the law then they are not the type of company you want to work with.

You may want to contact a lawyer about suing them for asking, but I'm not sure how that works.

The above applies to the US. If you are in a different country then you need to check your local laws.

Totally weird! Tell me more!

Is it a trap?