Do you think I’m wrong to say that sex education courses, regardless of whether they’re abstinence-only or more modern, typically already include admonitions against promiscuous sex?

No, you are not wrong to say that... but that’s not what your parent comment was saying. Your comment was saying that messaging did not have much influence on your behavior later in life. I’m saying that just because it didn’t have much influence on your behavior does not mean it hasn’t had much influence on the behavior of others

Well, if it’s widespread, and yet people are not behaving as we would wish, I would conclude it’s not very effective at influencing their behavior.

You do not know that the problem wouldn’t be worse without widespread messaging.

That's impossible to prove or disprove. Fortunately we don't have to because it's been well demonstrated that it's not sufficient.