Well, one way to not be completely surprised by their reaction when you bring it up is to talk about someone else (specific not made-up) that you have encountered who is poly - preferably someone who is making it work.
See what sort of reaction that gets. If you get a reaction like "These people are going to burn in the fieriest pits of hell, not only that I wouldn't want to even socialise with them", then you're subsequent conversation about YOU and your feelings would be very different that if they said "Wow, that's a cool concept, we should find out more!"
Obviously the true reaction is going to be something in between this, so you are going to need to read the non-verbal stuff carefully.
I'm not saying don't have the discussion, but maybe you need to do it in stages, not far apart....
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