evolution - Are there any structures in mammals that are used only by males?

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.