The distinction between organic and inorganic carbon compounds, while useful in organizing the vast subject of chemistry... is somewhat arbitrary. Organic chemistry is the science concerned with all aspects of organic compounds. Organic synthesis is the methodology of their preparation.
No, it is not possible to contract the typical STDs, such as syphilis, gonorrhea, chlamydia, hepatitis B, etc. from wearing someone else's dirty underwear. However crabs and scabies, may be spread in this fashion.
One type of underwear fetishism involves stockings. Underwear fetishism is a sexual fetishism relating to undergarments, and refers to the sexual excitement which some people experience from observing or handling certain types of underwear, including panties, stockings, pantyhose, bras, or other items. Some people experience sexual excitement from wearing certain types of underwear.