Concubines? Does god know about this?

Gen 25:6 says, "But unto the sons of the concubines, which Abraham had" He has multiple concubines? Really? And god knows about this? I hope not because in Gen 2:24 the bible says, "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." I guess this means they shall be one unless the man sees another woman of whom he would like to have himself known (sex.)