Upvote:2
Concubinage/cohabitation is a sin because, among other reasons, it
cf. McHugh & Callan's Moral Theology "2527. Forms of Fornication." (c):
(c) Free love is fornication committed with one's concubine, that is, with a woman who is not a public harlot but who has contracted with one man for habitual sexual intercourse as if they were man and wife. According to reports, this is quite common in Europe, where lawful marriage is very often preceded by free unions. The trial marriage advocated by some in this country, in which paramours agree to live together as husband and wife for a certain term of years or at pleasure, also falls under the category of concubinage. This sin is worse than mere who*emongering in one respect, namely, that it includes the purpose to continue in the state of sin, at least for a certain length of time. Moreover, there is often the public scandal and contempt for public opinion which other kinds of fornication may be free from. One who practises concubinage is living in a proximate occasion of sin, and hence he cannot be absolved unless he dismisses the concubine, if they cohabit, or agrees to keep away from her, if they do not cohabit.