A reader wrote after reading my post last week on genitalia support about my thoughts and experience running while wearing a jock strap. It is in an interesting query and an area that I don't have an experience. It has been nearly 20 years since I routinely wore a jock strap during exercise - running, playing basketball, and ultimate frisbee. Since then I have found the support and snugness of a running shorts liner to be adequate for race distances of 5K to 50K.
The goal for any running clothing, especially for long distances, is to minimize or reduce the impacts of friction. Friction from scrotum movement against the thighs can become unbearably painful. If considering a jock strap I would advise first using it in short practice runs and then wearing it for gradually longer training runs. If it works keep doing it. If it doesn't stop.
I also think part of chaffing risk management is moisture control. I have the most trouble with chaffing when my shorts become soaked with perspiration and or water. For an ultramarathon it might make sense to have a dry pair of shorts waiting, along with a stick of bodyglide, at a mid-course drop bag.
Ultrarunners, how do you minimize chaffing on long distance runs?