Why was having concubines not a sin like adultery? The Old Testament records many instances of men having concubines or many wives.  Why did having concubines not fall under the sin of adultery?

2 Samuel 5:13 - "After he left Hebron, David took more concubines and wives in Jerusalem, and more sons and daughters were born to him."

Genesis 25:6 - "But while he was still living, he gave gifts to the sons of his concubines and sent them away from his son Isaac to the land of the east." (Abraham)

I suppose the issue here must've been consent between all parties.
Having multiple wives was permitted (though not exactly endorsed) in the Law of Moses.

Adultery is having (or desiring) an intimate relationship with someone who's already married (to someone else). As it was forbidden in the Mosaic law, it was referring to a man and someone else's wife. It was not really speaking of a married man and another woman. Even then, though, the only occasion in which such a relationship is allowed is within marriage. So, in order for a man to have that kind of relationship with a woman, she would have to be a wife of some kind.

Concubines were essentially wives of lower status. They were still bound to the one man.