I always liked the Girl Genius explanation of leather underwear.
A perverted spark developed an compound that dissolved clothing, but left skin untouched. If the skin was dead, and what animal it belonged to didn't seem to matter. So certain characters have taken to wearing leather underthings as a precaution.
-Terry
-----
"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy
A perverted spark developed an compound that dissolved clothing, but left skin untouched. If the skin was dead, and what animal it belonged to didn't seem to matter. So certain characters have taken to wearing leather underthings as a precaution.
-Terry
-----
"so listen up boy, or pornography starring your mother will be the second worst thing to happen to you today"
TF2: Spy