For Christians, for the Church, such a reason is totally inadequate and totally inappropriate. More correctly, teens and unmarried persons should abstain from sexual activity because such activity is contrary to the purpose and will of God and is in fact sin.
When we tell people to abstain in order to avoid pregnancy we make the commandment of God of no account. This is utterly inexcusable for Christians, theologians and the Church.
The ‘practical’ reason for abstinence, the avoidance of an unwanted child, is a reason completely foreign to the biblical revelation. Not once will anyone read anywhere in any Scripture in any legitimate translation that the reason to avoid sexual activity is pregnancy. Rather, the reason is simpler and more profound: God commands it.
And why does God command it? Because the human body is made for more than just the satisfaction of desires. We are made to glorify God, yes, even in our bodies. To degrade that high calling and to behave as nothing more than a dog on the street in heat is to demean ourselves and treat contemptuously the image of God.
So, instead of telling teens (and unmarrieds) to avoid sex so they don’t have a child to care for- we are supposed to tell them to refrain from sexual activity because God wishes it. And his wishes supersede yours. Always.