Some studies show that sperm count is lower in males who wear tight underwear/clothing. It isn’t clear how this affects fertility (the ability to father a child), but it definitely doesn’t lower sperm count enough act as a form of birth control. 

What Is Sperm Count?

Sperm count is the number of sperm in semen, the fluid made during sexual excitement in males that comes out of the penis during ejaculation. Males with a normal sperm count have a higher chance of getting someone pregnant than those with a low sperm count. 

Why Would Tight Clothing Affect Sperm Count?

Sperm are made in the testicles, which are outside of the body. The testicles stay cooler than the rest of the body, at the right temperature for sperm to be made. If tight clothing/underwear push the testicles up against the body, the body’s heat might affect sperm count.

Will Wearing Tight Clothing Now Affect My Sperm Count Later?

Whether you wear boxers or briefs now is very unlikely to change your ability to father a child later. So wear what’s most comfortable for you.