The most common one is, very simply, most vaginas (or bums) aren't able to accommodate a penis much larger than average. Once you hit seven or eight inches you're going to run in to problems with hitting people in the cervix. If you're have a larger than average circumference then penetrative sex might just not be feasible. Some people can certainly enthusiastically tackle a large penis, but its certainly not everyone and can lead to people with large penises having difficult sex lives and relationships. When someone is experiencing difficulty having a fulfilling sex life with their partner, while at the same time being told their large penis is a symbol of sexual prowess and highly desirable, it can be very hurtful and upsetting. It's dehumanizing and doesn't acknowledge the difficulties people face in real life.

And there are other issues; If your penis is very prominent when erect than getting an unexpected erection can be effectively impossible to hide, putting you in a very awkward situation. A long penis might need to be held a certain way when sitting on the toilet to avoid hitting the water. You may have difficulty finding comfortable pants and underwear. The world is really not kind to people who deviate too far from the norm.