Most of the above posters have it right.
Irresponsible sexual behavior in teens causes physical, psychological, legal, spiritual, and emotional consequences.
Physically, teens run the risk of Aids, STD's, unwanted pregnancies, and complications of abortion when they are sexually active.
Legally, boys can have their lives ruined if they have sex with a minor (statutory rape), or are accused of doing so and the girl files charges.
Emotionally, especially for girls, it can lead to depression...as girls do still feel the stigma of being labeled loose if they are sexually active. After the fact, they may also feel badly that they are not virgins, as some girls really had it as a goal to save that gift for their future husbands.
If a teen is religious, premarital sex may be against their beliefs, so if they engage in it...they may feel they have betrayed their God.
So, while it may be "the thing to do", teens need to realize that irresponsible sex has many life-long consequences.