Grounds For Prohibition Of Zina
Well, a simple yes or no will help. Whatever be the reasons, sex with slaves is sex outside marriage. Now, if it were a universal moral construct, all societies would permit sex with slaves , but we know modern societies do not. My point here is not to discuss slavery.. but highlighting that sex with slaves itself is sex outside marriage.
Free sexual relationships are not inherently immoral, but Islam’s prohibition in this regard in the context of protecting institution of family is what makes this prohibition rational.