If you’re trying to conceive, do you need to change your underwear from boxers to briefs?
No doubt heat kills sperm. The testes live outside of the abdomen for a good reason. They need to be cool, about 4 degrees C cooler than body temperature. Testes that don’t descend, a condition called “cryptorchidism,” have real problems making sperm. Ovaries obviously don’t have this sensitivity to heat; they’re perfectly happy living in the abdomen.
The real question is: how much heat kills sperm? Crossing your legs increases your scrotal temperature by about 1 degree C, and it’s difficult to imagine that every man who crosses his legs is going to have a hard time impregnating his wife. Form fitting underwear, or briefs, raise it by a similar amount. Putting a laptop on your lap increases your scrotal temperature by about 1.5 degrees C or so.
So you probably shouldn’t worry about wearing boxers or briefs, but stay away from cooking your testes in a jacuzzi or sauna while you’re trying to conceive. Save that for stress reduction after the baby’s born.