skandalon
This post from Dappled Things (it's actually a link to the LJ feed, not to the actual blog) explains better than I've been able to the reason why I'm uneasy about living with Sean for any serious length of time before we get married. (In short, it's important for people, especially people who frequently seem to be 'good' Catholics, to avoid even the appearance of sin. This is both to prevent accusations of hypocrisy and to prevent giving a bad example - "if she does it, it must not be that bad" or "if she does it, then the Church must be ok with it")
Plus, I really like this example:
If I'm a practicing Catholic because Fr Schmidt is a saint and Sister Cecilia hasn't committed a venial sin for thirty years and the parish secretary is always so kind, then my Faith in Jesus is in sorry shape. When another secretary embezzles a hundred grand, and Sister Lucy starts running guns for the Sandinistas, and Fr Murphy runs off and joins the circus, where's my Faith then?