There was a story,about how one employee of Gabe Newell received him at home and stuff to discuss business or I don't know,and his kid shown him some monster concepts he drew that Gabe wanted to see. And apparently there was a suggestion of a monster in a phallic shape that would "rape" the player to death. So the guy thought Gabe would be like "K" but amazingly he laughed and they then spent the next hour discuting sexuality in monster design.
I thought that is where they got the idea for head crabs? Or something like that? Like, they wanted players to feel the panicky fear and unnerving feeling you get when you think of being raped. And so they started throwing around ideas for a rape monster, but the end result ended up being pitched. However, it lived on in spirit through the head crab taking over its host while the host is still alive and conscious, and keeping them in constant pain and misery. Or something like that.