You google the candidate. You find a webpage that matches. It talks about the candidates works for the gay/lesbian alliance. You don't hire the candidate because he sucks. He sues your company, alleging that you discriminated against his gayness.

In the specific case it is probably overblown, but HR fears aren't entirely rational.

Various states have laws that amount to information about the candidates past or current conditions that should not be used for job decisions. HR is there to protect the company, so they often will tell people what they can consider. Think of it as the human equivalent of not being able to comment on crime data when selling a home.