Answer: I believe it's the prostate.
Explanation:
Not 100% sure, but by this defenition of prostate, i'm more convinced it's this answer rather the others:
"The prostate gland is a male reproductive organ whose main function is to secrete prostate fluid, one of the components of semen. The muscles of the prostate gland also help propel this seminal fluid into the urethra during ejaculation"
So by that, we can see it pushes the semen through the body, whether by producing a liquid or force.
