Even if you aren't a Christian, even if you are an atheist, living together is NOT a good idea. Yeah, everybody is doing it now, it's the norm now. Who needs to get married when you have ready and willing people to move in with you? But, here's why it's not a good situation:
Well, when a couple gets formally married, they make a vow to each other. The vow is for "better or for worse, richer or poorer, in sickness and in health, till death do us part." right? This means that the couple is vowing to the other, not to themselves..It is a 'you' not a "me" situation. They are promising to take care of each other, be patient and loving to the other, stick with the other for all, and under all, circumstances.
As a person, no matter your belief system, we all need comfort, security, and love. This is not a matter of religious values, it is a matter of basic needs. We need to drive this into these young people's minds, we need to not be supportive of this kind of living arrangement, explaining to them that they are worth so much more than a selfish situation, they are worth a solid life of consideration and love and devotion higher than the superficial.