There's some research suggesting synthetic fabrics like polyester might affect sperm quality due to heat retention and reduced breathability, but calling it "complete infertility" or "as effective as birth control" is total BS.

The studies are limited and the effects aren't that dramatic. Cotton boxers vs polyester briefs might make a small difference in sperm count, but you're not gonna become sterile from your underwear choice.

Your take on polyester feeling awful is spot on though - it's basically wearing plastic bags.