it took OP several tries to get it through to Coworker that she was out of line and it seemed only to sink in half way at that.
I also like the referral to HR reply. But whatever the response, OP should make it fast and firm. I realize CW caught OP off-guard, but engaging in ANY further discussion with CW (even answer her questions) about the matter is a huge mistake. These types are slick. They will dig and dig and then carry twisted information.
For example: (this actually happened)
CW (who was relatively new): I hear that Nancy is having an affair with the boss and it's best to stay on her good side.
Me: (stunned) Hmm. That's a strange thing for you to say.
CW: What do you mean?
Me: Well, how is that anyone's business?
CW: (continues to blather)
Then later it got back to me that CW was going around saying "Portabella feels that Nancy and the boss's affair is no one's business." . . . .making it appear that I brought it up!!
In reprospect, I should have simply looked at her and not responded/reacted at all.
In OP's situation, any reply/discussion may come back to bite her.
CW: OP was talking about which employees are gay
Again: she is t-r-o-u-b-l-e.