There is quite a bit of information floating around on the Internet claiming that polyester and other synthetic underwear is harmful to our endocrine system by exposing it to micro plastics. Is there any proof to back up this claim or is it just another internet myth? On the same note, is it a bad for fertility to wear tight-fitting sport underwear (reduction in sperm production due to heat in testicles)?
You’ll have to ask the people making these claims for evidence, as it’s their burden to provide proof. If it’s asserted without evidence, we can dismiss it just as easily.
To your second question, the existing evidence is unclear on what, if any, causal role the type of undergarment plays in male fertility. Given that this has been studied for the better part of the past 30-years, I really wouldn’t worry about it.