Exo are a playable species in Destiny. They are described by Bungie as sinister, powerful, and tirless and are inspired by the undead, Master Chief, and the Terminator. They are one of the three playable Guardian races to choose from along with Human and Awoken. They are also by far the coolest race as well
The Exo began as war machines, they are self-aware machines built during humanity's Golden Age. The Exo's thoughts were erased and their protocol was reprogrammed after the Collapse to unite all sentient entities. They remain ciphers, even to themselves; their origins and purpose lost to time. Whoever built the Exos fashioned them in humanity's image, gifting them with diversity of mind and body. Inside the City, the Exo have the same status as the humans and the Awokens. They are widely regarded as siblings in arms
Because they were initially human to begin with.
How can exos be female or male if they don't have sex organs? They should be renamed to feminine and masculine.