Does what happened—the suitor kissing his fiancée and her sending him pictures of her body—while they believe they are married due to the intensity of their love, constitute a sin requiring repentance?

The fiancée is a stranger to her fiancé until the legal marriage contract is concluded with the presence of the guardian and witnesses. Therefore, if the legal marriage contract has not been concluded, the fiancé and fiancée remain strangers, and their feeling that they are a married couple or people's questions do not change this fact. One must repent from any past sins and avoid them in the future, for Allah is Most Merciful and accepts repentance.