So there is 100% agreement in this post, that open marrriage is bad, evil and destructive. Besides being sinful. People have given you Bible verses, too.
The thing is, up till about a generation ago, even non-Christians understood that this type of wickedness was sin. My parents were married 62 years when my father passed away. Never was this an issue. It was bad, wrong! Period. Yet my parents did not come to Christ till the last days of my father's life.
Which goes to prove that not only Christians but non-Christians understand how horrible what your husband is proposing, is. I hope you now understand that an open marriage is a deal breaker.
And as Lynn said, it is time for you to start reading the Bible, going to church, maybe getting in a Bible study and learn to walk with God. Being born in the church does not make you a Christian. Following Christ does. That is called being a disciple, and that is what Jesus told us to be and to make.