What to do about my parents?

I'm 22, I've been with my boyfriend for 4 years now. My boyfriend is very sweet, caring and loving. My parents make up lies about him all the time - they can't stand him. Recently I found out that my mom has been spouting lies into my 5 yr old nephews ear, causing him to dislike my boyfriend now as well. I don't know where they are coming up with these lies. My parents are pushing me away from them because it hurts ME to hear lies about the man I love. I'm 99% sure he's going to propose to me on Monday. I'm very excited about this.... and I'm 86% sure that my parents wont be happy for me which will cause me to be upset. Because of their hateful ways, I don't even want to tell them that I'll be engaged =( I've tried to talk with them about the problem - always end in an argument. I've confronted them on some of their lies - ends in more lies or covering it up. I'm at a loss. I feel that my parents are making me choose between him and them. Any advice is appreciated.