Regarding Sheldon's lack of any sort of sexual desire, don't forget about the exchange between Leonard and Sheldon in the first segment of the pilot episode. They go to a sperm bank to sell their sperm to earn extra money.
When the pilot episode is shown in syndication the first segment is omitted and the episode begins when they return home and meet Penny who is moving in across the hall. I guess the writers originally intended for Sheldon to have more desire for sex, etc. but as the show progressed there was some retconning of this part of his character. In the original unaired pilot there is mention of Sheldon having sex.