In my more naive, younger days I used to ask the annual Pride protestors a couple of questions. Not a debate question, or any kind of gotcha.
I'd just ask, "What is your goal here?" And they'd usually say something vague about making sure people know about Jesus and sin and forgiveness, yadayada.
And then I'd followup with, "Do you think this is an effective means of achieving that goal?"
nmw their answer was, you could always tell that they hadn't really thought about it in those terms.