What is the ruling on thinking about having intercourse with one's fiancée before the marriage contract, and what is the expiation for that, and does the fiancée become forbidden to the suitor?

You are not permitted to think about your fiancée with lust, as a preventative measure (saddan li al-dhari'ah) to avoid falling into what is forbidden (haram). The expiation for this is to repent to Allah Almighty by abandoning such thoughts and seeking forgiveness for them. This action does not make her forbidden to you. We advise you to hasten the completion of the legitimate marriage contract to protect yourself and to safeguard yourself from what is forbidden, and so that your fiancée becomes permissible for you.