I frequently go over to my friend's house on the weekends to work on projects. We usually meet up in the morning then work until lunch before we go our separate ways. My friend still lives with his parents who, while are nice, tend to be overly courteous.
Because of this, often when I'm about to leave, they insist I stay for lunch. While I do find the food to be good, there are often times I do not wish to stay for lunch. On one such occasion, I had made lunch plans with my significant other, and upon telling them this, they still insisted I eat lunch at their house. I was still going to be at their house for another hour to complete some work, so I wasn't sure how to decline this invitation. I ended up eating lunch there, then had to skip eating at my lunch date.
There was another occasion when I had a large breakfast, and by the time I arrived to my friend's house, much of his family has not eaten anything for the day. They invited me for breakfast and I told them I had a large breakfast and was not hungry. Despite this, they still insisted, and I had to force myself to eat a second breakfast.
There are times when I do not have lunch plans and very much enjoy eating there, but I also do not want to be expected to eat there every time I'm invited to. How can I decline having to eat there without coming as rude or making it sound like I'm not interested in an invitation in the future?
Edit: The location is in the US. The family itself come from a Pakistani culture.