Good picture diagram to explain: https://anatomy.elpaso.ttuhsc.edu/modules/pelvic_autonomic_module/Files/pelvic_page12p.jpg

Cavernous nerves are most commonly injured in prostatectomy. They are parasympathetic nerves that signal penile erection.

S2-S4

Penile urethra doesn't pass through the prostate. Prostatic urethra does. Dome of bladder is not near the prostate. Parasympathetic nerves are close to the prostate, hence more likely to be damaged during radical prostactectomy.