I feel for you pollyjean. My parents had a hard time dealing with it at first when I came out (not as bad as what you're describing though), but they learned to accept it over time and even be happy for me. I wouldn't try making ultimatums though, that doesn't sound like it would help in this situation. It's not something that you can force or rush, they have to decide for themselves whether or not they can deal with it. Just let them know that you love them and still want them to be a part of your life.
I hope that one day I can also say it got better for me over time.