I just wanted to reiterate this: If I don't romance my partner every single day, their romance goes down. I go to sleep with them very satisfied, but just sleeping through the night and going to work immediately deteriorates it. Also, I think all interactions (not just romantic) should work. I had them play chess, but this does nothing for the romance satisfaction.
Side note: having more sentiments for group or partner activities, such as chess, working out, karaoke, basketball, etc., would be nice. Similar to "cooking together."
I think you can get those activities to work towards romance if you set a date. So instead of just going to a bar to play karaoke together (as friends do), you could set a bar date. They get to date and work on their romance and sing karaoke at the same time.
I think the romance bar going down too fast is a bug, or maybe sims aren't compatible enough. I played a newly created sim couple when Lovestruck was released and that relationship didn't seem to deteriorate as fast as the one for a new sim couple I am currently playing. They woohooed a lot but still had time to pursue other interests and skills.
