Were concubines a sinful thing in Biblical times?
Yes but there were an allowed sinful thing and a redeemable sinful thing. Like many things God tells us what He intends and what the holy and righteous thing is, but then God also legally allows for it in order to curtail further sinful acts.
God doesn’t want concubines. But He understood the people’s culture and sinfulness. So He allowed for concubines to be had so that men would not just rape and abandon women or neglect their duty to child of concubines, etc. (which other cultures have often done).