When abstinence training is introduced into schools, they often REPLACE safe sex training. It's called "Abstinence Only" specifically. Just telling kids "Don't do it" isn't enough. Some of them are going to do it anyway, so they need to understand the realities of safe sex practices. Get tested, wrap it up, pop the pill, etc. In 2005, the federal government spent $168m in abstinence only programs. Congress did a study on these program and found that over 80% of these federally funded programs teach inaccurate information.
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It's when abstinence is taught along side responsible sex practices that one should expect the best results: lower teen pregnancy rate, lower abortion rate, lower STD rate.