I, sadly, have to refuse to tell my parents about my relationship. I came out to my mother about being bisexual three years ago and she has yet to accept the fact, so this is right out.
I keep slipping around coworkers though, and then they start asking questions. I have a very good sense that no one I work with would accept or understand it either.
I've found that the best way to go about telling people is this: talk about poly theoretically, get a read on them, and if they seem laid-back enough, press forward. Don't bother getting the close-minded ones that directly involved with your life.
I know that this doesn't help with the situation at hand. I wish I knew what to say. It's a tough place to be in, where you know one of the people who should love you unconditionally, doesn't. As for my mom, I do everything I can to scandalize her about my sexuality. The idea is that one of these days, she'll be desensitized enough to accept it. Again, this probably isn't the best example, because my mom isn't a normal human being. But eh, you do what you can.