Although starting a relationship with sex rarely leads teenagers into love, falling in love can often lead them into having sex as a physical affirmation and culmination of the emotional attachment they feel. One level of intimacy naturally leads to another, as all the love stories in all the movies they have ever seen has taught them to believe.
If your teenager is in a love relationship, he or she is much more likely to have sex. Therefore, if you see love blooming in your teenager's life, you definitely need to talk with him or her about what you wish for this relationship (that it be mutually respectful) and about what you don't wish for this relationship (that it become sexually active).
Besides the ordinary dangers of pregnancy and sexually transmitted disease, sexual activity between teenagers often brings them closer than their emotional intimacy can reach, and sex ends up frustrating, straining, and finally estranging the couple. Having sex only causes them to feel how emotionally far apart they really are.
The best advice you can give them is to wait before having sex — wait and give themselves time to emotionally grow together, time to nourish their emotional attachment before they complicate it with sexual loving.
Take the breakup of a teenage love relationship very seriously. Don't let a rejected boy turn hurt from rejection into anger and try to retaliate. And make sure a rejected girl does not slip from grief into self-destructive despondency from loss. And vice versa!