I think it's a Blue Dasher, based on the black-tipped tail. Eastern Pondhawks have white claspers. Do you remember what color its face was? The males are very similar-looking, face and tail aside and they're often found in the same areas.
IDK, lots of birds are even more sexually dimorphic--there's a parrot species that was originally identified as two species until they realized the green ones always mated with the red ones or something like that. And of course dragonflies are SUPER dimorphic.
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Blue Dasher: http://bugguide.net/node/view/598/bgimage
Eastern Pondhawk: http://bugguide.net/node/view/577
IDK, lots of birds are even more sexually dimorphic--there's a parrot species that was originally identified as two species until they realized the green ones always mated with the red ones or something like that. And of course dragonflies are SUPER dimorphic.