I have read that the most effective thing is stories— not statistics— in persuading people to be chaste. It works to say, “Well, you know what happened to Aunt Sadie...”. In our family, my son’s best friend was born to an unwed teenage mom. My daughter and I have talked about how, as a Christian, she gave life to her son, and we are all better off for it, because we have had the gift of his friendship. However, the ideal way is to bring life into the world with the full support of two responsible parents. So, she should keep her pants up until she is married, and most prepared to share the joy of children.
I think Christians forget that humility is part of the deal— so if it’s embarrassing to be caught sinning, oh well.
We have Jamie, our hairdresser, as our bad example. She's a very sweet person and can really cut hair, but having two children without marrying either father has not proved to be a wise life plan.
When we talk with the teens about how sex outside marriage is wrong and stupid, stupid and wrong, we can add, "You see how it's worked out for Jamie ...," and mention the latest catastrophe.