Well, I don't know how old you are, but it is possible that the sex education program you were taught is not the same one your kids are taught. You mention that you recalled being taught "the birds and the bees" in the fifth grade. That may or may not be the case now, but most abstinence-only programs do provide the basics on anatomy and conception, so kids are likely getting the information you remember learning.
Of course parents should be teaching their kids about sex, the biological aspect as well as the potential psychological, medical, emotional, spiritual, and financial consequences of engaging in sexual activity so that each kid can make decisions about sexuality with complete information. Note that I say "should." In an ideal world that is how things would work. We don't live in that world. Many parents choose not to educate their kids about these things. The kids still need the information though, so the schools are the natural choice to provide it. The idea that teachers are just too busy to teach students sex ed does not fly with me. This is critical information, the students have a right to know the facts and the teachers are there to teach them. The fact remains that there is a societal impact when teens are having babies, so it makes perfect sense for the educational system to ensure that what is needed is taught.
The problem with abstinence-only, besides that it doesn't work, is not that it "teaches about not having sex to prevent sex," the problem is that the programs tends to be filled with misinformation--out and out falsehoods--as well as value judgments about the characters of people who do not choose to be abstinent. Instructors use fear, lies, and good old-fashioned slut-shaming to try to keep kids from having sex.
Religion has a lot to do with it. So does politics. When the (religious) right stops trying to deny children access to the facts about sex and stops providing bullsh*t masquerading as education, people will stop blaming them for doing so.
And are you really suggesting that the onus is on kids to find out for themselves the facts about sex if their parents don't provide the information? Really? Think about that for a minute. I for one, don't want any kid having to rely on tv or movies for this information. What a failure on our part.
We have an obligation to educate our children--and I'm not just talking about our own kids, I mean we, as a society have an obligation to educate all children.