In 1800s era one-room houses, how public was sex? Would parents try to find a place away from the kids or was it acceptable to simply do it in front of everyone? In what era did the idea of sex being a private affair begin?
"Sex is a private affair" is one of the Human Universals. IOW, most people in most cultures strongly prefer to do it in privacy.
"One-room home" has been the dominant arrangement throughout human history for purely economic reasons. Believe it or not, it was relatively common in the late USSR for 3 generations to share a single room, often separated into semi-private annexes by sheets and/or wardrobes.
This contradiction was resolved by keeping sex to night time, when the kids were, presumably, asleep.