Question: What is the ruling on having relationship with one’s fiancee?
A: If what is meant by the saying of the questioner ‘before marriage’ before consummating it, and after the (Nikah) contract is made, then there is no sin, because after the contract is made, she will be his wife, even if it has not been consummated.
However, if it was before the contract, and during the proposal or before it, then it is forbidden, impermissible, because it is not allowed for a man to enjoy the company of a woman who is unrelated to him, neither by speech, nor by looking at her, nor by being alone with her.
This is because the Prophet ﷺ said: No man should stay with a lady in seclusion except in the presence of a Mahram. A woman should not travel except with a Mahram. [Sahih Muslim no. 1341]
– Shaykh Ibn al-Uthaymeen; Fatwa Islamiyah vol. 5, p. 122