Be willing to change your mind but not your morals.

From the dark and brilliant mind of Nietzsche came much truth, this is no exception.  We impose morals out of fear an immoral man will do what we are unwilling or able to.  That said in a relationship, you will have to change your mind from time to time, it is reasonable to expect that of your partner as well however you should never ask or expect your partner to change their morals.