What does the Church reprove of in a free union: the fact that they live together and/or have sexual relations?
The virtue of chastity must govern every relationship between a man and a woman: “Married people are called to live conjugal chastity; others practice chastity in continence” . Those who are not united in legitimate marriage must live perfect chastity, which includes not only abstaining from sexual relations but also from any expression of affection proper to marriage. If two persons of different sexes live under the same roof ‘as brothers and sisters,’ there would be no moral objection, except that this might be an occasion of temptation for them or a scandal to others, but if they live affectively as spouses, even if they do not have sexual relations, such a situation is morally wrong.
Fr. Miguel A. Fuentes, IVE
 Catechism of the Catholic Church, no. 2349.
Original post: Here
Other posts: I am not married, but I am good with God.