I&#x27;m not sure &quot;unlikely&quot; is the right word. There&#x27;s a lot of advice to men to wear boxers, not briefs, because &quot;briefs raise temperature&quot;.<p>This article (1999) says it&#x27;s not proved: <a href="http:&#x2F;&#x2F;edition.cnn.com&#x2F;HEALTH&#x2F;men&#x2F;9906&#x2F;14&#x2F;men.infertility&#x2F;" rel="nofollow">http:&#x2F;&#x2F;edition.cnn.com&#x2F;HEALTH&#x2F;men&#x2F;9906&#x2F;14&#x2F;men.infertility&#x2F;</a><p>This article gives two reports that did show an effect: (2015) <a href="http:&#x2F;&#x2F;fivethirtyeight.com&#x2F;features&#x2F;men-those-tightie-whities-really-are-killing-your-sperm-count&#x2F;" rel="nofollow">http:&#x2F;&#x2F;fivethirtyeight.com&#x2F;features&#x2F;men-those-tightie-whitie...</a><p>Having something warm in your pocket, which also makes everything a bit tighter, may increase temperature a bit.  And raised temperature might be bad.  So it&#x27;s not smoking gun, but it&#x27;s not totally implausible.