Actually there are penile muscles. Look up Kegels. They are done by squeezing your urinary and anal muscles to "hold it" if you have to go to the bathroom, such as to stop the flow of urine. Squeezing these muscles will make the penis move, but they're surrounding it, not inside of it.
Anyway, the sphincter is what stops urine, not the body of the penis itself. There is one right at the end of the bladder and another within the length of the penis. If you can 'hold it' for a little when you need to urinate, then you will not become incontinent. Just don't go every hour, let those muscles do a little work to stay strong! At the same time, don't go hours and hours needing to use the bathroom and not doing it, because that would be bad for your organs.