Have you read any of the posts about people coming out as poly, Jlpanian? I'm all for doing so, but it can be a rough ride. Might help to read up a bit first.
It probably will help that its still theoretical, you're asking them to deal with a concept, not a concept + a person.
I have no experience coming out myself (yet!), but I do have lots of expertise dealing with angry people, especially family members!
Key things to remember for me are to make a big effort to stay calm when attacked. To wait silently to respond until there is quiet, no matter how long it takes. Then to quietly state that I'm here to talk about something, not to be attacked. I rinse and repeat as necessary, and stay prepared to leave if the attacks keep coming, or if I'm in danger of losing my cool completely. Words have much more weight if the speaker stays in control.
Hmm, I guess that was unsolicited advice... not to mention it's what works for me. Doesn't mean it will work for you.