STJOMO:
Any ideas on what the church says about this type of circumstance? I’m convinced it’s no more acceptable than living together and being sexually active. What kind of solid information can I give her that would convince her that what she is contemplating is wrong (other than a dad’s advice)? I’ve searched the Catechism and Catholic Answers. I’ve only found references to chastity. She says that that isn’t the case here.
A couple of quick thoughts. Ask her if she would like to marry someone, someday, and if so, would she like that marriage to last a lifetime?
If she is honest, she can’t very well answer that she is hoping for a marriage to break up.
Then follow-up with the point that cohabitation increases the probability the current relationship will break up. It also increases the risk that any eventual marriage to a new, as yet unknown, spouse will also break up.
Chaste or not, there is a seriousness to living together that has a long term impact. If one makes the assumption that your daughter and boyfriend have made a serious emotional comittment to each other (obviously serious enough to defy her parents and live with him), then I am reminded of a quote from Janet Smith:
Most people have been lied to at some point. Most people have made promises, and broken those promises, and had promises made to them, and those promises have been broken. And they’re marrying someone who has lied and made promises and had promises broken, and they don’t trust each other quite as much. They don’t even trust themselves quite as much. “I’ve said these things before, I’ve made these commitments before–can I keep them? He’s said these things before, he’s made these commitments before–can he keep them?”
As someone who went through a number of “relationships” prior to marriage, one way I put it is each failed relationship left the emotional equivalent of “scar tissue”. It was very difficult to get past the “scar tissue” and learn how to live a fully-committed married life. The doubts that Janet Smith mentions kept arising in the early years of my marriage. Thank God we had the help of the Holy Spirit to make it through.
Also, you asked for Church teaching. If you enter the following search term in google, you get a LOT of Church teaching on the subject: “cohabitation site:usccb.org”. The US Bishop’s conference has 23 documents discussing cohabitation. They are well worth reading.