Dan Savage has really good advice for telling people about things that might really challenge them. Carl might find this useful. Basically, if you say "I'm poly and I have a second girlfriend... it's been going on for a while but I didn't know how to bring it up before" and you say it in a tone of voice like you're saying "I have cancer, it's late stage and I couldn't bear to say anything earlier" people will respond badly. Tone of voice and wording is everything. This is such a new idea to most people, they just don't know how to respond. If you say it like it's bad or shameful, they'll take that in and act like it is and may not be able to change after that.
So, say it like it's the awesome thing it is! Like, "There's something that's been making me really happy, and I've decided it's time to share it with you. The gf and I decided to open up our relationship a while back, and I've been involved with a wonderful girl for X months. Everything is out in the open, everyone's doing good, and I love them both to death. If you have any questions, please feel free to ask, I know it's unusual but it's been so great for all of us." Or whatever version of that is true to your situation.
Some people will respond well or poorly either way, but for many it's all in the presentation.