Does it only happen with a provider or also with the SO?
Please remember that most (if not all - I am not a medically licensed person) human males are born with their testicles INSIDE the abdomen. At some time of your adolescence (early puberty?) the testicles descend into the scrotal sac and the corresponding openings in the groin area close up, not always totally.
Although besides your question, there are adult males whose testicles have never descended into the scrotal sac; I don't know if that condition, called "undescended testicles", of necessity also means an inablility to procreate.
Now, you want to check that out with your doc.

Re:Shy balls

Here, kitty, kitty Polecat,

You need to relax your cremaster muscle! Retractile testes result from hyperactivity of the cremaster muscle that causes retraction of the testicle into the inguinal canal. With gentle manipulation, your balls can go into the scrotum, where they will stay when the cremaster muscle is relaxed.

Here's a textbook explanation of that wondrous muscle you may not know you had: The cremaster muscle is a muscle that covers the testicles. Its function is to raise and lower the scrotum in order to regulate the temperature of the testicles and promote spermatogenesis. In a cool environment the cremaster draws the testcles closer to the body preventing heat loss, while when it is warmer the cremaster relaxes allowing the testicles to cool.

The interesting thing is that the psychoanalytic literature documents a case of cremaster muscle activity and ensuing retraction of the testicles during intense dream activity. So it seems that those spasms could be the result of a little case of "nerves?"

So, my dear cat in heat...you can visit a urologist for a more detailed investigation, or you can ask yourself if you are at all nervous or tense before engaging with a provider. Is it the same provider over and over, or new ones all the time? Maybe you need to relax and feel a little more comfortable?

On the other hand, from an aesthetical perspective, many women find a small scrotum with two tiny balls inside very sexy and attractive. It's cute and signals youth, vim and vigor. Look at all the statues of the male ideal from the antiquities. The whole genital configuration is always depicted like a beautiful flower, like a piece of delicious candy, ready to be nibbled. Believe me, it evokes a sense of ravenous rapture in women. But of course, if it bugs you, you might want to engage in some relaxation exercises before doing the nasty. Or, as stated before, make an appointment with a specialist to find out if this is something that can't be corrected.

Meow to you,
the Love Goddess