Of course it is morally unacceptable - one should not have sex slaves to begin with!
NEITHER MUHAMMAD NOR HIS GOD DISAPPROVE IF A MUSLIM MAN IS COMMITTING ADULTERY WITH ANOTHER MAN'S WIFE, IF THIS SECOND MAN IS NOT A MUSLIM.
Muhammad and his merry men treated the women of Awtas in a similar manner to the women of Mustaliq, as these 2 hadiths record:
Abu Sa’id Al Khudri said “The Apostle of Allaah(ﷺ) 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 Apostle of Allaah(ﷺ) were reluctant to have intercourse with the female captives in the presence of their husbands who were unbelievers. So, Allaah the exalted sent down the Qur’anic verse “And all married women (are forbidden) unto you save those (captives) whom your right hand posses.” This is to say they are lawful for them when they complete their waiting period (Sunan Abu Dawud 2 2150).
Abu Sa'id al-Khudri (Allah be pleased with him) reported that at the Battle of Hanain Allah's Messenger (ﷺ) 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, 1456).