I would have a serious and pointed conversation with.your hr dept. This isn't workplace harassment as you overheard a private conversation, still I would not come out in that environment.
I know that this kind of benighted view isn't uncommon, for instance I remember all too well a co-worker venturing the opinion over lunch many years ago that he'd rather his son be dead than transgender. I'll never forget the casual anger he expressed, seemingly not knowing he was speaking to a trans woman.
I'm glad to be out and finally transitioning in a workplace that's not perfect and still the most accepting I can imagine. I hope you will be met with better acceptance than you were forced to overhear.