"Clearly the person who accepts the Church as an infallible guide will believe whatever the Church teaches." — Saint Thomas Aquinas.
I think this is a great and yet simple and to the point. This is what a true Catholic needs to live by. People who call themselves Catholic but don't beilieve what the Church teaches are cafeteria Catholics. They pick and choose what they want to believe because they think they know better than the Church. Not everything the Church teaches is easy, but we are called to abide and live in accordance to what it teaches.
If you accept the Church as and infallible guide then you have to accept that abortion, euthanasia, and divorce are all grave sins, and that they are all a part of the culture of death. Far to many people call themselves Catholic and yet believe those things are ok, politicians not excluded.