You may want your parents to think or to behave in a certain way but you can’t force them to. You have tried all that you can. You have suggested therapy. They went but now are refusing. It is likely a futile effort to continue to suggest therapy.
When people won’t change their behavior, then you must change yours. In your case, this may mean limiting your involvement or interaction with your parents. If they say hurtful things, then be around them less often. Perhaps you should also be less open with them about your thoughts, opinions and values. If your values no longer match theirs, then realize that continued discussion of controversial topics will likely lead to an argument. This may be the unfortunate reality. You may have to find a different way to interact with your parents.
To become a psychologically healthy adult one must become an independent thinker. In practical terms, this often means no longer automatically sharing the values or opinions of one’s family of origin. Independent thinkers form their own ideas about what is right and wrong for them. It is vitally important that you feel free to develop your own ideas. It’s not only okay to do so but it is necessary for your continued growth. There can be fallout associated with becoming an independent thinker. Sometimes the family of origin is offended by a new way of thinking. It can be seen as threatening and detrimental to the family. Not adopting the values of the family of origin can sometimes be viewed as a rejection of their values. Some family members become resentful and angry. It is possible that this is what is happening in your family situation.
If your family refuses therapy, then I would suggest individual counseling. In individual counseling you can learn a new way of interacting with your family. Since they won’t change, it is apparent that you’ll have to and counseling is the best way to learn these skills. Please take care. I wish you the best.