Allah says it’s ok to sleep with married slaves.
“Abu Sa’id al-Khudri (Allah her pleased with him) reported that at the Battle of Hanain Allah’s Messenger (Peace be upon him) sent an army to Autas and encountered the enemy and fought with them. Having overcome them and taken them captives, the Companions of Allah’s Messenger (may peace te upon him) seemed to refrain from having intercourse with captive women because of their husbands being polytheists. Then Allah, Most High, sent down regarding that:” And women already married, except those whom your right hands possess (iv. 24)” (i. e. they were lawful for them when their ‘Idda period came to an end).” Sahih Muslim 8:3432
Abu Said al-Khudri said: “The apostle of Allah sent a military expedition to Awtas on the occasion of the battle of Hunain. They met their enemy and fought with them. They defeated them and took them captives. Some of the Companions of the apostle of Allah were reluctant to have intercourse with the female captives in the presence of their husbands who were unbelievers. So Allah, the Exalted, sent down the Quranic verse, “And all married women (are forbidden) unto you save those (captives) whom your right hands possess”. That is to say, they are lawful for them when they complete their waiting period.” Sunan Abu Dawud 2:2150