It’s normally not a problem at all even if you literally never masturbate, due to erections that occur during sleep.

No. That's not true. But it can shrink if you never get erections. But I wouldn't exactly call getting an erection during sleep "using it"

Someone has already said that it's the muscle you use to stop peeing midstream, but I wanted to add a few other points:

• Once you've identified the muscle, you shouldn't do kegels while peeing, because this can lead to UTIs and an unhappy bladder

• When you do kegels, you should try to only engage your pelvic floor muscles; try to avoid engaging other muscles in your thighs, buttocks or abdomen. Isolating your pelvic floor muscles can be tricky at first, but it gets easier

• As well as improving aspects of sexual function, kegels also help with incontinence. If you practice them now, you'll be thankful for it as you age

You know how you stop peeing mid pee? That's how. Clench. Hold it for a count of 10. Release and repeat. Do it in the morning and evening to start.