I agree with Donk, with just one exception. I know very well that a large penis can readily burst a zip fly ... not to mention fly buttons. This used to happen to me quite regularly in my student days, and until I was more careful to get trousers with more room where I need it. It especially applied to the tightish cords we used to wear in the early 70s. Believe me, they were not good for guys like us.
I can still feel the pain.
Disaster might happen suddenly (as with a very powerful and quick erection), or, what I think was worse, I might gradually become aware of the slight breeze, and had to begin wondering just how long I had been walking around looking like that.
For a while I had stronger zips put on my trousers as a matter of course.