I spent 12 some odd years of my life teaching sexual and reproductive health to teenagers. I'm going to tl;dr this by giving the punchline: talk to your kids about sex early and often. That said...
I actually got into the field of reproductive health because I had a paralyzing fear of public speaking — I figured throwing myself into talking to adolescents about sexual health would break me of my fear and do some good at the same time. It very quickly became something that i was professionally and personally invested in. My family and friends were simultaneously weirded out and impressed. My father found roughly 3,000 condoms in my room once and pulled me aside to say "you know I may be the only father who can be proud when they find that many condoms in their daughter's room." He thankfully decided not to mention the zillion packets of lube.
As I grew more comfortable with giving the sex talk to groups of teenagers (6th grade to college), it evolved. Word got around that I would answer pretty much anything I was asked and that I was casual and funny about it and boy did they ask. In the end, prodded by my bosses and the schools, I changed my class from sex ed to something more along the lines of Sex Q&A. A simple, comprehensive talk on STIs, condom use, and healthy relationships became much more. Once I started taking write in anonymous questions things really got interesting.
So I was no longer only answering questions about "what time can you have sex to be sure you won't get pregnant" but also "how do you have anal sex without getting hurt," "what do you say to a girl after the first time," "can a condom be too big," and "is there something wrong with me if I masturbate every day?" They want to know how to keep themselves safe (those questions thankfully never go out of style and are, rightfully so, the focus of any talk) but they want to know how to keep themselves sexually healthy and in healthy relationships beyond condoms and birth control.
What's my point? Some of these kids, kids who have been having sex for years with multiple partners, are dangerously uninformed. They don't understand consequences, they don't understand how to protect themselves and they don't have anyone to turn to for guidance besides their equally clueless friends. That is unacceptable. It's a problem we should be addressing when they're young, before sexual activity begins, before they think they know all the answers. They have so many questions that they are only finding online with answers like "douching with mountain dew will keep you from getting pregnant," and "you can't get an STI from anal sex," and "you're a bad person if you have sex with more than one partner." They hide things from parents and doctors because they are afraid of being judged. They have questions and they want answers - they just need the right person to talk to.
I can't remember having the sex talk with my parents. Not the "where babies come from" conversation mind you, but the actual talk about sex. I certainly didn't hear about it in school. I might have learned it from my friends, or picked it up from reading books like Forever, Endless Love, and Flowers in the Attic or seen a super special episode of Blossom on the topic. This is probably not the best way to figure things out.
So if you are a parent, an older sibling, a teacher, or a person who is in a position to shape young minds, talk to them. Answer their questions. Parents, acknowledge that you are uncomfortable too but are there to talk anyway. Answer questions even if they don't ask. Cover your bases, don't just whip out a banana and talk about condoms. Remember all the questions you had to fumble through and broach them. We are too quick to assume they are learning these things in school.
If you're not comfortable with this, find someone who is. Buy them a book, point them at a reliable website. Because rates of teen pregnancy and STIs and HIV and misery are too high and education is one of the easiest ways we have to go towards changing that for the better.
Edited to include: please please talk to them about condom negotiation because that is one of the most important, most challenging, and least taught arts involving sex!