This was addressed fairly recently over at the EWTN forums, as well as here on a couple of threads regarding scandal.
Basically: Two unmarried, opposite sex people living together are: 1. In a situation which is a near occasion of sin. The temptations to impropriety at least and grave sin at most would be overwhelming.
2. Causing a situation of scandal. In this culture, at this time, whether it’s a “right” or “wrong” assumption, people assume that such a couple, especially a couple who were dating, engaged, or otherwise known to be “a couple”, if living in the same apartment or house are engaging in sexual activity. The fact that such a couple would automatically be saying “but nothing is going on” shows that the immediate first assumption by Joe or Jane Average is that something IS going on. Especially if a couple identifies themselves as practicing, faithful Catholics (by going to Mass, etc.), again the tacit, implicit assumption is that Catholics have no problem with sexual activity, living together, etc. before marriage. And that is a scandal, because the Catholic Church does, indeed, have a “problem” with sexual activity or even the appearance of sexual activity before marriage. Paul specifically warns us in I Corinthians not to be “barriers” or hindrances to other people. He warns us not to harm the faith of others. Scandal harms.
3. In addition to scandal, such activity only reinforces the current, secular climate where men and women are reduced to objects of desire or lust, rather than love. True love involves a respect for the beloved. Living together, even “chastely”, is disrespectful of the other person, and of one’s self. There is absolutely no situation where a non-married couple would “HAVE” to live together. It is charitable to consider the temptation one would be putting on one’s beloved, the scandal and the loss of character repute or “good name” of the beloved, and the weakening or rupture of the person’s Catholic faith and thus the possible loss of that person’s soul for eternity.