بسم الله الرحمن الرحيم
Nikah does not take place by the ijab and qubool of boy and girl without witnesses. They will not become wife and husband for each other even after such ijab and qubool. Therefore, their sexual acts and intercourse will remain unlawful and haram. It is mentioned in Majma’ al-Anhur (1/472, pub Darul Kutub al-Ilmiyah Beirut):
وشرط ایضاً حضور شاھدین الخ ومثلہ فی الکتب الاخری من الفقہ
It is wrong what you were told that nikah is validated by intercourse. There is no such provision in Shariah. The one who told you so seems to be ignorant and transgressor, may Allah grant him hidayah.
Allah knows Best!
Darul Uloom Deoband