If you try and apply a rational lens to an irrational worldview, it's going to end in tears. But basically they don't want anyone to fuck until marriage, and they believe (or at least claim to believe) that abstinence-only sex ed is best for this. It's essentially just the tough on crime philosophy applied to sex instead of drugs. Anything they say about STDs or Teen Pregnancy being the real reason is just posturing not to look like religious hicks, as actual sex ed obviously outperforms their bullshit. Also as a side bonus for these assholes it hurts queer and trans people to give bad sex ed.