There are three things that must be considered in this scenario. Actual sexual sin, the** near** occasion of mortal sin, and the possibility of scandal. Actual sin and the near occasion of sin are a little easier to deal with. Either the couple in question sins mortally or they don’t and the situation will either very strongly tempt a person to mortal sin or it wont.
Scandal is a little more complicated to figure out. There are a lot of factors to consider here. How much of a moral authority figure are you to other people? Just the fact that you are a Christian or a Catholic makes scandal more important to avoid as any scandal you cause will have a more lasting effect. People can then think “well, if even the good religious people are doing it then it can’t really be that bad to do”. If you are in an even higher level of authority, such as a religious youth leader, a nun, a priest etc the importance of avoiding scandal is even greater.
Another factor is how likely the action you are taking is likely to encourage others to sin. And I’ve got to say, on this count, people living together for monetary reasons is not nearly as scandalous as it used to be. Especially if the couple in question is fairly young, who are they going to influence besides their peers and other younger people? But, people in their teens and twenties have become much more accepting of the idea of people of the opposite sex living together without having any kind of romantic relationship. So the fact that two people are living together for monetary purposes isn’t nearly as likely to lead them to believe that a sexual relationship outside of marriage is ok as thats less likely to be what they assume is going on. Add to that the fact that most people that age assume that if you’ve been dating someone for more than a few weeks you are having sex it seems like the scandal of leading others to think sex is ok outside of marriage is more attatched to dating itself than to sharing a house for monetary reasons.
However, having said that, if you were in a relationship where it is well-known that you are being chaste and waiting for marriage it is again more important to avoid entering a living situation which might cause other people to think you are actually not being chaste. Thats because if it is actually known and believed you are chaste then your relationship becomes a public example of morality for others to look up to, an example that such a thing really is possible, and so it raises your level of moral authority and the importance of avoiding scandal.
Another thing that comes into play is how much the living situation looks like a cohabiting situation rather than a platonic arrangement for money. For instance, if it was a guy and two girls living together when the guy is engaged to someone who lives elsewhere that is going to be a lot less likely to cause scandal then if it is a guy and girl engaged to each other who move into a house with just the two of them. As I noted, its a freaking complex subject.
This is especially when you bring the principle of double-effect into play. Which principle states that an action which will have both good and bad consequences it is moral so long as the bad consequences are not intended and are proportionate to the good consequences. So, on top of the question of what scandal actually is and to what extent it exists in different situations you add in the complication of needing to balance the bad effect of scandal against the good effects of living in the same house. Such as the good of having a roof over your head etc.
