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There are examples of structures which only serve a purpose in females, but it seems like the opposite could also be true. Are there any structures which exist in both mammalian sexes and only serve a purpose in males?

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I would speculate the prostate. – leonardo May 5 '12 at 18:29
but it seems like the opposite should also be true. Why? – nico May 5 '12 at 18:43
@nico because a structure that becomes vestigial in females and still serves some purpose in males will have evolutionary pressure to remain intact in both sexes. Likewise, a mutation that serves a useful purpose in only half the population will still be a useful mutation as far as the population is concerned. – Shep May 5 '12 at 18:47
@Shep: my point is the fact that an example of a shared structure that is used only in females exist does not in any way imply that the opposite should also be true. It could, but there is definitely no causality here. – nico May 5 '12 at 20:00
@nico ah, should --> could, I didn't mean to imply any causality there. – Shep May 5 '12 at 20:35

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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&qid=1336286206&sr=8-1

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Females of Rangifer tarandus have horns which only used by males in competitions. Other species of deer only have males with horns.

Apart from this, possibly, clitoris and vulvar lips have no purpose in women: the vulvar lips are underdeveloped halves of scrotum, and clitoris has no need to be so sensitive as it is because it plays no role in sexual reproduction in females.

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