One reason Juliet gives for delaying her marriage to Paris is that it is too sudden. She says she wonders why her father is being so hasty in demanding that she marry Paris. In addition, she really hasn't even met Paris yet; he has not courted her. As she states in her lines, "I wonder at this haste, that I must wed / Ere he that should be husband comes to woo" (III.v.121-22). Aside from feeling that her match with Paris is being too hastily made, she also argues that she does not yet feel ready to become a bride. Not only that, she claims she dislikes Paris, even though she really hasn't met him yet. We see both of these arguments in her lines:
I will not marry yet; and when I do, I swear
It shall be Romeo, whom you know I hate,
Rather than Paris. (124-26)
For all of these reasons, she begs her mother to postpone her marriage to Paris for "a month, a week" (208). However, alone with Nurse, Juliet proclaims her real reason for wanting to delay the marriage to Paris--she is already married, and she knows that marrying a second man would be considered a sin. We see her proclaim how her religious convictions are adding to her distress about being forced to marry Paris in the her line, "My husband is on earth; my faith in heaven" (214).
Hence, the reasons Juliet gives to her parents for delaying the marriage to Paris is that she does not feel ready to marry and she does not like Paris; however, the real reason is that she is already married.