Tight pants, in particular, can lead to urinary tract infections, over-active bladders, and even twisted testicles. When it comes to sperm however, the problem isn’t the constriction that tight garments cause; it’s the heat.
The testicles function best when they are slightly cooler than body temperature. That’s why they hang outside of the body. Tight underwear or pants push the testicles up against the body, subsequently increasing their temperature. This increased temperature causes reductions in sperm count and can also decrease sperm motility.