As GR and I cope with having sex while our grown kids' bedrooms are on both sides of our room, we are constantly reminded to keep the noise level down as much as possible when the girls are home. And we're all in our bedrooms with walls between us and doors closed. He and I play music or TV to help muffle the sounds we make during sex. Still, they hear things that they would rather not hear.
Back in the old days couples often lived in an open-style, one room log cabin. I envision the parents bed off to one corner and kids sleeping in the loft. I am sure they did not always have bedrooms with walls and doors. How did the parents have sex without the kids watching and hearing them? Sure, they could have hung blankets around their bed for a fair degree of visual privacy but did they always resort to "silent sex" for audio privacy? How did they do each other?