They didn’t, really.
If the word “sex” was ever even mentioned, mom would go bonkers God bless her, she just doesn’t want to think about her baby growing up.. Dad, being a devout Christian, as well as a healthcare worker, took a rather awkward, but slightly more constructive approach. He basically told me, in timid tones, that he had sex before he was married and that I shouldn’t, but if I do, I should use a condom, but I really shouldn’t anyway and if I had any questions I should ask him. I didn’t. He also told me, for some reason, that girls would still like me even if I wasn’t tall, blond, and muscular.
My paternal grandmother gave me a book called “What’s Going on Down There” that talked about how dick size doesn’t matter lol and that sex is messy, but fun, and that STDs (illustrated as little monsters and trolls) could totally fuck up your day if you aren’t careful. Problem is, she gave me this book when I was 17 about the same time I had started experiencing the joys of vaginas first hand, sans penile penetration. That didn’t happen until I was 20.
Most of my formative experiences with sexual knowledge, however questionable they may be, came from a guy named Eric in elementary school. He was a real hoodlum, pushed back two grades from 5th grade into my 3rd grade class. He told us all as we sat in stunned silence that if you didn’t masturbate daily, your penis would turn green and fall off, that you had to put your entire package in a girl’s vagina for sex to work, and that the average penis was 15 inches long. He was eventually expelled for bringing a pair of boob ear muffs to school that the little blue haired cafeteria ladies discovered.
Being a smart kid, I pretty much figured everything out for myself, despite the fact that nobody really talked to me about it truthfully. At least I developed mostly normally, I suppose.