"Only thing that is 100% effective is not having sex at all.

"
Thanks to the USA's experiment with abstinence only education from 2001 to 2008, we know that it is not true. Abstinence has a pretty high failure rate.
The past decade has shown that good intentions are not a good birth control method. Abstinence is 100% effective with perfect use but for normal use, it gets pretty close to withdrawal in normal use. Recent studies have shown that abstinence postpones first time sex by 3 months, an abstinence pledge postpone it by 12 months but the net result of it is a slightly higher pregnancy and abortion rate and a much higher STD infection rate.
In some European countries, the combination of hormonal birth control and condoms are taught to young people. (abstinence is taught as a lifestyle choice, not a birth control method.) They have an almost non-existent unwanted pregnancy rate in those countries (0.5 per 100 vs. 5.5 per 100 in the USA).
(Sorry ClearR, I know you did not mean it this way but it still is worth it to look at the facts around the 100% effective abstinence only claims.)