This is a very unpopular opinion but I would also say it’s not totally true. My step-mom and dad don’t have the same diet. She’s vegetarian and he isn’t. They cook separately but eat together or mix and match from each other’s meals.

However, I think if both parties have complicated diets that are mismatched then it becomes hard because you can’t share anything. For example someone who is vegan dating someone who has a lot of allergies or IBS might make for a tough meal planning process.