And if they say, “Why?”—and I’m dealing with a situation like this right now, where a kid is about to move in as a Christian with his girlfriend (“Not going to have sex!”)—here’s the why:
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.
And the last thing I might say is to the guy: “Sixteen years from now—it’s going to be here just like that—your daughter says she wants to move in with her boyfriend. What would you say? She says, ‘Dad, it’s their problem! We’re not going to have sex!’ What would you say? Well, say it to yourself now.”