In terms of telling my parents, I want to echo what some others have said. I would not tell them unless we were in a committed, stable relationship for at least a year. No need to ruffle their feathers unnecessarily, especially since telling my mother that I was bisexual resulted in her suggesting, "It's just a phase. Don't worry, you'll grow out of it once you get a little more experienced with life."
I have tested my mother out by bringing up polygamy and polyamory in general and gauging her opinions through her comments. Seems she is quite opposed to the idea that anyone could ever love two people equally or to the same degree. I think she is the "one true love" kind of person, and having another person ruins the supposed "magic" of a partnership.
My father is very unlikely to accept anything so radical. He often makes homosexual jokes about his friends as derogatory remarks, so I have never told him about being bisexual and probably never will unless, like I said, we get into a committed, stable relationship with someone who will come with us to holidays, live with us, etc.
Regardless of how my family feels about it, I have many loving friends who are either part of the poly community or just really great open minded people who love me just the way I am. Even if I can't talk to my family about it and share our relationship experiences and the ups and downs of a poly lifestyle with them, I still have that wonderful "true" family of friends who will lend an ear when things are exciting or have gone sour.
I think that is really what telling your family is about anyway, having someone accepting who will listen and offer advice/support when you need it. If you can find that outside of your parents, it is my opinion that you don't need to tell them, especially if it would just add stress or tension to your family life.