Tight-fitting underwear for men (briefs, underpants, or underclothing) gives more support to the scrotum and testicles and can avoid discomforts, injuries, or testicle torsions, notably when doing sports or other rigorous activities. It can also supply general support for sensitive testicles so that they don’t move around too much, as such avoiding possible pains and discomfort.
Another — certainly not unimportant — reason for men to wear tight underwear is simply out of aesthetic motives; they think it looks sexier or they feel sexier and more attractive. In view of both comfort and aesthetics it can also be the better choice to wear tight and slim-fitting briefs under one’s trousers and not a wide, loosely fitted boxer short.
Of course, all good things we do in excess can become a bad thing, so constraining the testicles too tightly may also cause discomfort or pains of the scrotum, testicles, or groin area. And although tight underwear can avoid rubbing and irritation it may also cause ventilation issues and sweating in the groin area, which again can favor things like jock itch or yeast infections.
Another thing is that wearing tight underwear is associated with lower sperm counts in men. It’s thought that it increases the temperature of the scrotum, which affects the production of sperm by the testicles. Mind that in normal circumstances the temperature in the human testes is about two to three degrees Celsius lower than the body temperature, creating the most suitable temperature for sperm production.
In any case, some research has shown up to twenty-five percent more sperm cells in men who usually wear loose fitting boxer shorts, so that may be the better choice for men who try to have children. On the other hand, research on the topic has also concluded that there seems to be no correlation with lower testosterone levels for those who wear tight-fitting underwear.