Juliet understands that her marriage vows were very important and she cannot break them. When her father proposes that she marry Paris, she tells him she will not. To just the Nurse she says "My husband is on earth, my faith in heaven. How shall that faith return again to earth unless that husband send it me from heaven by leaving earth?" This means that she is bound not just by law but by faith as well to Romeo and would only be free to marry someone else if Romeo were to die.
