There is no contradiction. You are free to believe anything you want and worship any way you choose, but if you want to practice Catholicism, you are bound by the rules of that religion.
Let's put it another way: could someone consider themselves a Republican if they believed in a centralized government-controlled Communist economy?
Hmmm... Let's pose that question to the whole forum - I'm sure we could come up with quite a few elected GOP officials that a consensus of FReepers would feel meet that exact description.