What is the biblical argument against having a concubine?

It is generally accepted that "having a concubine" = "not socially accepted." However, 1. Solomon had 700 wives and 300 concubines. 2. Having concubines was mentioned in the Old Testament. 3. I cannot find any New Testament verse against having concubines. Thus, the question: What is the scriptural evidence against a man having a concubine?