I've got this problem as well. Tested on every family I've made since the game came out, and some of these have the most comfy furniture available.
Esp when feeding them it becomes a problem. A sim will go to the fridge, grab a plate, and stand there eating. Then if I tell another Sim to fetch food or make food, the command will instantly cancel (without them complaining at all, mind you) like the fridge no longer exists.
When playing games on the TV, socializing, or doing pretty much anything that normally would include sitting, they just stand there and refuse to sit down unless I tell them to do it.
With larger families, making sure the fridge isn't constantly blocked becomes annoying very fast and the easy workaround is simply to have a fridge for every member of the family.
Oddly enough, if one sim makes a group meal and the others get a plate from there, usually they'll find somewhere to sit as long as there's a free seat by a table. They don't seem willing to eat in the couches anymore.