Well, at least in mosquito-fish (Gambusia holbrooki). A recent study shows that female mosquito-fish select their mates based on the size of their genitalia. Females tended to 'associate' more with males with slightly larger 'gonopodia' (I'm no ichthyologist, but I suppose it's the fish near-equivalent to a mammalian penis). However, this distinction was only made when females had to choose between large males; for small males, gonopodium size had no effect.
What I think is kind of neat about this study is it is an example of sexual selection that actually involves the sexual organ; most other traits in sexual selection studies are things like bird feathers, deer antlers, and the like. I don't think, however, this female-choice criterion of plays a major role in mammals. But if it does, perhaps this is the true reason why we wear pants?
Kahn AT, Mautz B, and Jennions MD. Females prefer to associate with males with longer intromittent organs in mosquitofish. Biol Lett in press.