What is the ruling on having a girlfriend (in a living relationship)?

Question 8: A man lived with a woman (without marriage) for a long time and she bore him many children. Afterwards, he legally married two women who bore him many children. Now, the man wants to drive the first woman out of his house, but the children will not allow him to do so. What should he do?

Answer: Firstly, having a girlfriend is considered Zinaa (sexual intercourse outside marriage), which is prohibited according to the Book of Allaah, the Sunnah (whatever is reported from the Prophet) and the Ijmaa` (consensus of scholars). These two individuals must separate, perform Tawbah (repentance to Allaah) and seek Allaah’s Forgiveness. If they sincerely repent and conclude a sound marriage contract, there will be no sin on them… read more here.