Greg publicly identified as heterosexual up until about 2013. Fans and viewers noticed he listed himself as "pansexual" when he re-launched his Onision forums. He did not discuss this change publicly and some viewers assumed he did it as a joke. One of the main reasons people believed this was because Greg stated many times he was not attracted to men.
In 2014, Greg began saying he was pansexual on his social media. Confusingly enough, he would flip-flop between telling his fans he is heterosexual and pansexual. Once even saying he was hetero, only to go back to saying he was pan in less than a week. (Source)
In June 2015, many people began questioning Greg's pansexuality even more when he uploaded the video "Why Do People Like Futanari and Twenty One Pilots". In the video, he complained that he did not understand how futanari, or hermaphroditism, was erotic. He adds that if he was bisexual, he would probably think it was hot. In the same video, he explains, "I'm pansexual, but that mostly means I'm heterosexual."($ource)
When viewers commented saying they believed Greg does not know the definition of pansexual, he would brush them off and sometimes reply with jokes.
On August 14, 2015, Greg uploaded "Is Onision A Gay Married Dad? (+ Pansexual)", where he revealed he realized he was in fact heterosexual. He explains he thought pansexual meant "if my wife grew a dick I'd still love her." ($ource)