Board Thread:General Discussion/@comment-24100805-20160201152915/@comment-32182236-20180316130533

It being uncertain is the reason why "they" is a neutral point of view in the first place. If they had one, canon gender, that would be used. But, of course, they don't. Just like they're not canonically good or evil. (They could also be neither here too-See:Neutral Route.)

Think of it less like you physically controlling their gender, and more the way you would think of Genocide Frisk vs Pacifist Frisk. Or pretty much any other game where you get to make choices for your character. Or the reason why Frisk fell underground. There's no true canon here, that's something we have to decide for ourselves. And no matter what gender we choose, we still got it right. Because the whole point is it's another choice for you, the player. Frisk is you. You roleplay Frisk.