Many male runners enjoy wearing boxer briefs or trunks as these are going to help preventing chafing. If your legs touch while you run, tighter and longer boxer briefs will be your best bet against chafing.
Should I wear underwear when running?
To reduce the chances of infections it is best to wear underwear that is made of more natural fibres or has sweat-wicking properties, or run without underwear. The looser your running shorts the better it is for reducing the chances of moisture and bacteria build up.
Why do runners wear underwear?
Many athletes however wear compression (lycra-type) underwear beneath these shorts for better support. In certain events, male and female athletes instead tend to wear tight-fitting lycra shorts. These are most common-place to sprints, hurdles, pole vault, and jumps but may be worn for distance events and throws, too.
Are you supposed to wear underwear in running shorts?
Underwear/Liner: Running shorts have a built-in liner and the rule of thumb is that you don't wear underwear with them because the extra layer of fabric can cause uncomfortable chafing.