Say a woman is married 10 years with children, and falls in love with someone else. In order to leave the marriage, she would have to sunder all kinds of relationships and face all kinds of disapproval. It would be very expensive, the fallout from longtime friends and family members-in-law would be huge. I can see her staying because of the fear of the fallout for leaving. Would you call that marriage a prison? Would you call marriage a "cult"?
That's because you are seeing this nun as married. She would be commiting adultry by leaving in her mind because she is taught she is married to Christ. But that is nto true. We all give our life to Him for salvation... Her vows to God as His will not change regardless of her environment...once they give their life to him...it's His. They are not unfaithful to Him by leaving a cloisture. Their relationship to God will remain...and actually grow and develope once they do leave.