Is sexual fantasizing about one's fiancé forbidden (haram), and what is its expiation?

The suitor is a stranger to his fiancée until the marriage contract is concluded. Fleeting thoughts are not held against you, as long as you do not dwell on them. However, intentional fantasies are forbidden except with one's husband, and their expiation is repentance. The Permanent Committee for Issuing Fatwas has stated that it is not permissible for an unmarried young man to fantasize about intercourse, as it is a means to fall into evil.