Help Finding a Story Featuring Indestructible Balls, Nurses, and Thumb Tacks

I remember a few details of a story I read a while ago, but now I cannot find it again. The basic premise was:

> Guy has a dominating GF, she convinces him to go to the doctor's, they tie him up and inject his balls with a wonder serum to make them impervious to damage, while also more sensitive to pain. They go home, and the GF slaps, stomps, hammers, and even sticks thumb tacks in the invulnerable balls. There's a part at the end where the nurses come to check up on him.

If anyone knows this story, I'd love to know where I can read it again.