How does a mans pelvic floor muscles get overtight or overused?

Almost impossible. Men have difficulty even knowing how to exercise the pelvic floor muscles, so an overuse injury seems to me to be impossible. If you think you have this, it is probably something else.
And no, a lot of sex wouldn't cause the pelvic floor muscles to be tight or overused. Generally, sex is a very good thing for men. But if you are having symptoms down there, best to get checked. Good luck!