from the Bible Encyclopaedia, via Google -

Concubine
in the Bible denotes a female conjugally united to a man, but in a relation inferior to that of a wife

Among the early Jews, from various causes, the difference between a wife and a concubine was less marked than it would be amongst us. The concubine was a wife of secondary rank. There are various laws recorded providing for their protection (Ex. 21:7; Deut. 21:10-14), and setting limits to the relation they sustained to the household to which they belonged (Gen. 21:14; 25:6). They had no authority in the family, nor could they share in the household government.

The immediate cause of concubinage might be gathered from the conjugal histories of Abraham and Jacob (Gen. 16). But in process of time, the custom of concubinage degenerated, and laws were made to restrain and regulate it (Ex. 21:7-9).

Christianity has restored the sacred institution of marriage to its original character, and concubinage is ranked with the sins of fornication and adultery (Matt. 19:5-9; 1 Cor. 7:2).



Don't think it would have been my first choice for an existence, but if there was a scarcity of males - eg after a series of wars - I guess it would provide a socially acceptable role for women. Presumably only those capable of supporting them, took on concubines.

an interesting thing I've noted is in conservative western society, especially in conservative Christianity, although having only the one spouce, what is pictured as the ideal wife is not the Biblical discription of a wife but the Biblical discription of a concubine.