It’s generally pretty hard for liberally-minded folk to understand bigotry based on sexual orientation, nationality, or skin colour. We tend to think we should be able to reason with bigots and convnice them that their fears are unjustified. I think that’s largely wrong.
I think one of the best ways to understand bigotry is to imagine you’re talking about consensual incest or bestiality, things which I don’t think are morally wrong, but find viscerally disgusting, rather than homosexuality and interracial marriage, things towards which I have neither moral objections nor visceral disgust.
Another way, which is was reminded of while watching the (fucking excellent) District 9, is to actually think of reviled groups as non-human and consider how you’d react to them. For my money, D-9 does the best job of this I’ve seen.
Here’s the 2005 short film on which D-9 is based:
Whenever you wonder how Nazis could treat Jews as badly as they did, imaginatively replace Jews with Cthulu-like tentacled things and think again.