John Piper addresses the question whether it’s OK for a Christian couple to live together if they aren’t married. His answer, in part:

It’s not primarily, “You’re going to be tempted, and you’re going to give in, and you’re going to have more sex. That’s why.” That’s not the main reason.

The main reason is that when a man and a woman live together it says crystal clear to the world that having sex together without marriage is okay. That’s what it says.

Now, you say you’re a Christian. Do you want to say that sex before marriage is okay? And if you want to say that, then something is profoundly wrong!

And if you say, “That’s their problem,” you’re not loving people. It’s not their problem. It is your problem. You should take steps to communicate truth, and the sanctity of sex in marriage is a glorious truth, and you should want to hallow it and cherish it.