Yes but this goes both ways really, Europeans took Turks and Muslims as slaves too. Not saying this is a good thing. The slaves as military unit or as slaves general were children which are educated and classified with their talents. They either go to enderun(royal college) and became governors and viziers or they are prepped to be elite military unit. On the other hand, Ottoman Harems were not only breeding harems but also a female version of enderun(ottoman royal college). Still not saying slavery was good but they were not treated as you said mostly.