I went to the doctor to ask him what the best way to avoid pregnancy is. He said "Orange juice". This surprised me, as no doubt it surprised you. I didn't know that orange juice is an effective contraceptive.
I asked him when to drink it: before or after sex.
"Neither", he replied, "instead of."

The moral of this story is that abstinence is the only 100% guarantee. If you are having sex, you have to be mature enough to be responsible for your actions.