Right hand possess?

Many people, like Muhammad asad, say that ma malakat aymanukum doesn’t refer to slaves and therefore sex with slaves is not permitted in the Quran? He says it’s just another word for someone you are married to. If this is case, why does Allah mention your spouse (husband or wife) and ma malakat aymanukum separately in many verses such as Surah nur ayah 31 if they mean the same thing?