I just sorta... figured it out? I guess? It was less that I knew exactly how it worked from the start, and more that I slowly figured more of it out as I went, without ever being entirely wrong. As far back as I can remember, I knew it involved two people of opposing gender doing *something*.
At that point, it may as well have been a business dealing between them, for all I thought about it. I gathered, from the reactions I got when I asked, that the matter was supposed to be a private one.
The rest was pretty much filled in by sitcoms. Never had any sort of talk with my parents.