Is being preoccupied with a girl, wishing to marry her, while not neglecting the remembrance of Allah, considered a sin?

If the preoccupation of thought and the attachment of the heart to a girl is beyond the young man's will, then there is no sin upon him, unless it leads to something forbidden, such as unlawful gazing, or prevents an obligation, such as neglecting prayer. In such cases, he must take steps to divert himself from it. However, if the attachment and preoccupation result from forbidden causes, such as unlawful gazing and seclusion (khalwa), then it is unlawful and one must repent from it. The young man is advised to marry the girl if she is religious; if he cannot, then he must avert his gaze from her. Furthermore, he should not let his thoughts be captive to a creature, lest he deprive himself of intimacy with Allah.