Years ago I had a sex only gal that would visit off and on. She would often say she could feel my balls hitting her, I liked hearing it.
Different girl, I was doing her from behind, she reached under and grabbed my balls, that got me off pretty quick and she held on as I unloaded, was hot.
Different girl, I asked her to hold my balls while I was pumping her, she squeezed too hard and it hurt but was exciting.
Present day, wife and I enjoy mfm fun with a guy we know semi regularly. It's damn hot to watch my wife hold his ball sack. It's hot to see his balls swing as he's doing her. All very exciting.