I lived with my husband for two years before we were married. We'd been engaged for a few months before we moved into our new apartment together.
My uber-Catholic father was less than thrilled - but it's my life, not his.
A friend of mine got around her own uber-Catholic father's disapproval by "living" in the dorms while really
living in an off-campus apartment with her fiance (now husband - they've been married nearly 10 years). Her father thought she was a sweet innocent Catholic girl not living in sin; she got to live with her fiance and have a place to crash on campus if she needed it; and her "roommate" essentially had a huge single. Win-win-win.
I recommend living together before marriage, but only if you really think your relationship is headed toward marriage or a similar long-term commitment. Don't do it if your relationship is rocky.