Is thinking about the marital relationship with one's fiancée, merely as an imagination in the mind, considered forbidden?

Imaginations and thoughts are among the self-whisperings (hadith al-nafs) that are divinely pardoned as long as they are not spoken or acted upon. However, one should not indulge in them excessively, lest they lead to falling into the forbidden (haram). The suitor (khatib) is a stranger (ajnabi) to his fiancée (makhtubah); therefore, it is not permissible for him to be in seclusion with her, or touch her, or converse with her except out of necessity. Romantic conversations between them are forbidden.