I have wondered this for a long time, actually. Most parents today take for granted some degree of privacy, and this leaves time and opportunity for sex. Based on the rate of births in most of history, it seems like a ton of sex must have been happening. But how?
Were kids just witness to it? Were kids out playing in the woods alone? Did ma and pa sneak off periodically during the farm labor or whatever?
Obviously I know the answer would vary by time and culture and social status. But this seems like a fundamental part of human life that just gets ignored in most discussion of history. Especially in the very puritanical societies, I don't really understand how they maintained the birth rates they did. Surely someone wrote accounts of this at least in passing?
Edit: spelling
You would probably be interested in Histoire de la vie privée- French series that among other things give long paragraphs about changing degree of privacy.
Let's focus on Europe and Middle Ages. One must remember that for the most of the history sex wasn't taboo. When I started to read sources about Middle Ages family life I was shocked how much attention was paid to sexuality in both ways- priests were very vocal about evils of lust, indirectly leaving probably the most detailed descriptions of sexual behaviour and rituals. Weddings were all about sex with proper songs , jokes and guests waiting by the door and reminding the couple that their parents long for grandchildren. And don't even start with the Midsummer nights that were the pain for the Church long after they managed to baptise everyone in parish. In Poland, the impotence of the husband was one of a few situations that couple could be granted divorce. How was wife's complains validated? Local prostitute lend a hand, so man could prove that he is capable of having sex. In the presence of reliable witness- read: kins of the both parts, priest and leaders of the community.
Remember that people back then lived together. Physically together- common bathing, eating, working and having sex. No wonder that their border of privacy were different than ours. Have you ever read journals of Europeans in Japan who couldn't undress themselves in common bath, while for the locals it was the most natural thing possible? Past is a foreign country:)
Separate bedrooms are quite new discovery. Lords slept with their whole courts, families shared bed, most of the cottages had one big room for everyone. Were children witness to martial sex? Sure. And couple's parents, siblings and everyone who happened to sleep in the same room. And that wasn't anything to be ashamed of since marriage was about fertility and the last known example of virgin birth was soo long time ago :)
You could also take a look at this thread for information that seems related to what you're asking