>and underpants, how? because of how tight they are?

Temperature control is the reason the testicles are in a vulnerable unprotected spot outside of the body. If they were inside the rib cage they would be better protected but warmer.

Underpants that keep them warm are working against this.

Nudism at home is weird but works. Works wonders on the testo level as well if the wife does it too. Avoid hot beverages.