I believe sex other than between a man and woman married to each other is sin,
This is your belief, but that belief should not be taught in schools. I'm ok with teaching abstinence is the only 100% way to prevent pregnancy and STDs (because its true), but that should be followed up with teaching if you DO have sex, use protection. I'm not of the Christian faith, therefor I believe sex between two (or more) willing partners is fine. If its just sex for the sake of sex with someone other than a long time partner, you need protection from both STD's and pregnancy (where it applies). That is what should be taught in schools.