Are there any structures in mammals that are used only by males?
Some researchers consider the neural circuitry responsible for orgasm vestigial in women. I disagree with this for a number of reasons, but here is a book exploring the hypothesis:

http://www.amazon.com/The-Case-Female-Orgasm-Evolution/dp/0674022467/ref=sr_1_1?ie=UTF8&amp;qid=1336286206&amp;sr=8-1