Great post. I broadly agree with your line of thinking.

a) Does preparing adolescents for sex increase the likelihood they'll engage in it? - Absolutely not.
b) Do STD infection rates make you strongly inclined to discourage your (possibly hypothetical) kids from engaging in casual sex even if protected? (Do they make you strongly disinclined to engage in casual sex?) - yes, but not 'discourage' in a preachy way
c) Should the public interest and public health concerns trump parental rights to shielding kids from sex ed content? - Absolutely yes