There is a cozy sofa ... !

Direct them to the sofa and they will sit down and watch it.

you know , i'd like to see the Sim do that on its own !      

Mine do it usually 90% of the time.

you are lucky ! My Sims seem to have a subscribtion to malfunktion !!

My Sims also stand up watching TV most of the time, as well as standing up while eating.
It's like they are constantly in a hurry, and don't have time to relax.

If you have the Parenthood Game Pack, have your Sims set the dining room table and Sims will automatically go and use the dining table to eat their meals.

Thank you for letting me know, I do not own the Parenthood Game Pack, so as for now, I play with a strict behavior-control of my household, making sure that sitting down is part their behavior.
And I actually think they may be learning, and I can see a change in their behavior, hehe.