Listening to Dan Savage today. In an older podcast, he's expounding on Bristol Palin's statement (no, not the one where she says abstinence is unrealistic) that abstinence is the only 100% foolproof way to prevent teenage pregnancy.
To which Dan Savage says:
Which is also a completely 100%, foolproof way to prevent pregnancy. Thank you Dan.
He also lists a number of other things that people can do that are not abstinent, but are also not penetrative vaginal sex, and therefore would also completely prevent pregnancy, but I'll let you to your imaginations for that one.
It's an extensive list.