To me the cause is obvious - the longer you're together, the less you try and establish what you like and dislike. Your partner puts a plate of bacon in front

To me the cause is obvious - the longer you're together, the less you try and establish what you like and dislike. Your partner puts a plate of bacon in front of you and you eat it : they assume you like bacon. You might tolerate bacon but not like it, but you eat it because it was prepared for you and there's no reason to say 'I don't like bacon'. Same goes for movies : you just accept watching movies with your partner that you don't particularly care for, but you do it because you're spending time together. The partner assumes you like that movie because you went along and didn't complain. In other words, it's likely that the study miscontrues 'like' for 'doesn't actively dislike'. I can watch romantic comedies without complaint, I don't like romantic comedies as a genre. It's just easier to eat the food, watch the movie than it is to start a federal case over it. After all, if you've been in a relationship for a while, chances are there might be kids, and you probably just relish the ability to watch any movie in peace and quiet.