Urine is produced by the kidney through the ultrafiltration and
selective reabsorption processes and is stored in the urinary
bladder.
Until the pressure is too big and you'll feel the need to
urinate. Then it will be released from the bladder and travels
through the urethra to the penis.
Sperm is produced by the testis and is stored in the epididymis
(inactive form) before it enters the sperm duct.
It travels through the sperm duct to the urethra. A gland,
seminal vesicle opens to each sperm duct.
The area where the sperm ducts join the urethra, there's the
prostate gland and Cowper's gland.
The 3 glands will secrete a fluid known as semen that provide
nutrients and enzymes to nourish the sperm and activate them.
So semen + sperm and urine do not flow through the urethra at
the same time because of the sphincter muscle at the base of the
urinary bladder.
Erm.. O.o