The phrase "only abstinence is 100% effective" is a slogan for teens, not a claim that only abstinence only sex education is 100% effective (which is clearly false).
>There is room to mock it on a personal level, by pointing out that we are willing to engage in far more dangerous activities without 100% guarantees on safety. 'Seatbelts fail, only not driving is 100% effective'.
It depends on how much a person wants to avoid having an abortion or unplanned pregnancy, and how much value they get out of sex.