I know it sounds cliche, but I'd like to give your dad the benefit of the doubt that he's just acting how he feels is best for you, given his past experiences and society's brainwashing. It's understandable how, without understanding poly, he imagines his little girl is on a path to heartbreak.
I also think there's a difference between "accepting you" and "accepting your choices." If it was you that he truly didn't accept, he wouldn't be willing to talk to you and support you and be there for you down the road.
None of this is to say that he's right, just that he's not out to get you. I completely understand how his reaction is really hurtful and how you feel rejected. That's completely normal. His ignorance and unwillingness to learn about your choices is unfair.
Is the stepmom willing to see your side of things? Maybe you can establish a rapport about your lifestyle with her, and she can help your father understand?
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker