STD's are serious, infertility can be caused by STD's , not to mention HIV which if it develops into AIDs will kill.
Chlamydia is becoming more common and in women it can lead to PID which can cause ectopic pregnancies, sleeping around isn't "cool" and if you can't keep your dick in your pants at least have the sense to protect yourself and the other person.
The worst is when a partner cheats and thier partners end up with something that can completely mess up thier lives.
Think with the head on your shoulders not the one on the end of your dick.

Unwanted pregnancies are no joke either, too many dumb fucks out there with kids by people they don't even want to be with, contraception is both peoples responsibility.