The question was how to put together a rational argument; so those who refer to Biblical quotes or sin either missed the question, or stopped too short.
I assume they already know what the Bible says, though it is worth reminding them, especially some of the quotes from Paul. However, if they are really intent on living together, what they need is to understand why it is a sin. Nothing we do sinfully is done in a vacuum; we live in community, and sin impacts that community.
Some of the best information is being developed by the secular research, particularly in Sociology. The findings are amazing, and the statistics astounding.
Intercourse releases certain chemicals, which act to bind the couple. If they are bound without the attendant intent and promise of fidelity, that chemical binding puts them in a push/pull emotional format; a half in/half out way of living. Then, when one succeeds in convincing the other who lacked committment, to marry, they in a relationship which takes staying power for tough times, without having developed the staying power. Why were they living together without marriage? Someone wasn’t ready, if not both not ready. Sex was good. Their friends were all doing it. She was afraid sh’e lose him (she should be so luck at this stage, instead of unlucky at a later stage!). They were too immature to make a committment. They wanted to “try it out”.
The old addage that women give sex to give love, and men give love to get sex, has altogether too mush truth in it.
And another guess is that they have already been engaging in intercourse, and realize the lie they are living; and think that living together is “more honest”.
The information and statistics are out there; it just takes some digging. And while you are at it, you might visit an analysis of Theology of the Body, which gives a very positive view of what marriage really is. Good luck!
