I think this is where some middle ground is needed.
Personally, I'm in favor of an abstinence first program. Tell kids that the only 100% effective way to prevent pregancy and/or spreading of STDs is to not have sex, BUT that if they do there are ways to reduce the risk of unwanted consequences. Then go into the different methods.
What bothers a lot of conservatives isn't the teaching of sex ed, but the fact that there some teachers and organizations who are zealots in the other direction. IMO, this is one area where a middle ground would be best.