Greek Slaves

As i was reading the entries about greek slavery i found it very interesting that somewhere so bountiful with money and power was so full of slavery, i mean this in regards to their princples, thousands of temples with prostitutes present, man or animal?, rule or be ruled. A slaves "body minister to the needs of life", the needs of their masters life. The reading mentioned how money was being made and how ships captains were taking advantage of the system, "not for every man is the voyage to corinth." How was all of this justified?