But you can break a penis, and it's a lot more common than you'd think. The break is actually known as a
penile fracture, despite the penis lacking a bone, and your professor is actually wrong and made you look foolish for nothing. Knew this girl that did it to a guy on accident once, she thought her period had started early in the middle of sex. Guy started to realize things hurt, and sure enough, broken penis. She ended up with the unfortunate nickname of "Cockbreaker" for the rest of her life.
Edit: Should have read the rest of the thread before responding.