system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when men ejaculate they do so in "pumps", but when they urinate, its in a constant stream?
gpt
Generally different mechanisms and muscles. Urine is stored in the bladder, which is basically a big muscular bag, and more or less just needs to be released.

Ejaculation is a more complex process that includes using the muscles of the pelvic floor to drive the semen out. Basically, the semen is stored in tubes that connect to the urinary duct, and then these muscles propel it out (see a textbook for details).

Interestingly, you can also use these same muscles to propel out remaining urine in the urinary duct. 

As for *why* this is the case, it simply makes reproduction easier by "firing" semen toward the womb. This is done most efficiently in waves of muscle contraction. This is not really a requirement for urination.