Help! I Disagree with My Parents
The key here is to engage in disagreements with greater balance and respect than you did when you were a child. If you're treated like a child, respond like an adult. If you're treated like a child, it can be very easy to react accordingly—getting defensive, withdrawing, saying hurtful things, etc. Parents need to be reminded that you are an adult, and the best way you can demonstrate that is to behave like an adult. Roles during these transitional years may sometimes be awkward, confusing, or stressful. Be patient.
Don't flaunt your choices that run counter to their value systems. If you make a decision that you know disturbs your parents, respect that they do not agree with your viewpoint. If you are living with your parents, realize that this arrangement especially requires actions that create respect on all sides. Be willing to discuss your decisions and be open to their input, but don't continue to bring up the issue or try to change their minds. Let your actions of respect and love move your relationship forward, even when you strongly disagree.