Can I just add something to the discussion?
I'm not a teen (most of you know I'm an adult anyway) - but unprotected sex before
OR after marriage can lead to pregnancy... even if you are happily married you don't necessarily want to have a baby... you may not feel ready for parent hood, you may not be able to afford a baby, you may just want to spend some time with your partner without the huge, long-term committment that a child brings to a marriage...
Whether you are married or not...
protected sex (whatever kind of birth control you choose) is sensible if you don't want to get pregnant.
I exlude from this any people who will not use birth control for religious reasons... before anyone mentions it ^_^
Otherwise, the only time you should ever have unprotected sex is when you are trying to concieve a child. At all other times both before and after marriage, use contraception. It's not "rocket science!"