Is it bad if you wear somewhat think underwear, along with shorts and stuff, in the summer... can it do any serious damage
Wearing warm underwear
As Rad says, usually the effect is just to reduce your count of healthy sperm - a problem if you are trying to achieve pregnancy, but not otherwise. (But not reliably effective enough to use as a method of contraception.) When your balls get cooler again, your sperm count will go back up.
However, if they are kept too warm permanently (for years), you do increase your chances of testicular cancer.