Abstinenence only is absolutely ineffective.

After studying 510 abstinence education programs on adolescent sexual activity and related knowledge and behavioral outcomes:

"The impact findings show no overall impact on teen sexual activity, no differences in rates of unprotected sex, and some impacts on knowledge of STDs and perceived effectiveness of condoms and birth control pills."

(emphasis mine)

There you have it. We can all tell our sons and daughters to abstain from sex until marriage, but it will have no impact, none. Looks like we will have to try something else. Duh!

Here is the nine year study. Conclusions are on page 83 of 164.