Why were Muslims allowed to maintain sexual relations with slave girls during the Prophet’s time, and not anymore?
Answer
Slave girls were a problem Islam faced at the beginning. The Almighty didn’t eliminate it overnight because it could have caused more harm. He arranged for its gradual elimination by encouraging people to free them voluntarily, and later, ordered that all slaves and slave girls who wanted to be freed should be allowed the facility if they promised to pay their masters their price. Finally, the option of the prisoners of war getting enslaved was withdrawn for Muslims. Thus, the door for making new slaves was shut forever.
During the time when slavery was allowed, there were laws in place to deal with them. It was one of those laws that allowed masters to have sex with slave girls.
For more details please see “Islam view on Slavery and Slave Girls”.