Concubinage is an interpersonal relationship in which a person engages in an ongoing sexual relationship with another person to whom they are not or cannot be married. The inability to marry may be due to a multiplicity of factors, such as differences in social rank (including sex slave status), an extant marriage, religious prohibitions, professional ones (for example Roman soldiers) or a lack of recognition by appropriate authorities. The woman in such a relationship is referred to as a concubine. Historically, concubinage was frequently voluntary by the woman or her family, as it provided a measure of economic security for the woman involved.While long-term sexual relationships instead of marriage have become increasingly common in the Western world over the last few decades, these are generally not termed concubinage.