Jeez, never knew people felt so strongly about shorts in F (or J as someone said). I've definitely flown F in shorts, but I always dress smartly, not anything baggy or tacky. I tend to get hot in cabins, especially in J and F with actual blankets, so pants/PJ pants get too uncomfortable for me. Interestingly, I've seen plenty of people in F not wearing shorts but looking like they just rolled out of bed (loose T, sweatpants, etc), so IMO it's all about presentation. If you can wear shorts but still look neat and proper, by all means go for it.

But at the end of the day, you do you. You're paying for comfort in F/J, why do something that's going to make you uncomfortable.