This theory got started at Episode, "We Need to Talk" where Greg tell Connie that if she need to talk with ANOTHER Human Being about Gems, he will the one to talk to and she five hand him but Steven says this while rubbing his gemstone
"Human Being"
This could interpret as Steven question his own humanity.
Even in Chille Tid where Steven have a dream where him and Crystal Gems are humans, did he views Crystal Gems as Human or he want a normal life as a normal human?
Remember the old saying, "Which sides are you on?"