Frisk: Look, I can't even tell the bathrooms apart!

Asriel: We just wait for somebody to come out and then you'll know.

Oh come, come now. It's simple, open to interpretation, whatever gender you think Frisk is is the one that fits for your personal play through.
I thought of Frisk as a boy and Chara as a girl for some interesting duality as I played through the Pacifist Run,
It can go both ways, Frisk is a girl and Chara as a boy, which makes their relationship with Asriel VERY interesting.

The argument about which gender Frisk is is dumb, because they're a character meant to be interpreted any way you see fit. What's dumber is the argument that Frisk identifies as neither.

It becomes dumb when they argue so vehemently for Frisk being a transvestite or gender nothing that anyone who wants a more liner option gets yelled at to the point that it turns them off Undertale.

It's fine if they just state that's what they believe and its cool if you think otherwise, it's not when they make it so anyone who likes Frisk and Chara to be more set genders will be yelled at them for daring to break their precious headcanon that Toby Fox never said was right.

I should've been more clear on that, I apologize if I offended.

I'm still annoyed that a friend of mine who loves the game enough to write stories for it got yelled at for making Frisk a boy.

I see Frisk as a hot blooded boy trying his very best to save everyone he can and whose determination and courage sees them through their journey. If they're questioning anything when they get older, it's why they couldn't save Asriel properly. That's the beauty of Undertale, it allows you to interpret everything differently.

It's not that the headcanon is dumb, when you force it upon others and say it's the only one that should be canon, that's when it becomes dumb. I think the argument is lame since it's a game where you can form your own conclusions and as all of them are valid as the next, meaning there is no canon beyond what is firmly established in game, saying one above all others is right is in poor taste.

TL:DR, don't pass off your fanfiction as the sole canon and force other people to like, most of whom who just want to enjoy a really good game and form their own conclusions about it.

There must be a third determination option.

Trying to figure out what gender you are,
it fills you with determination.

For some reason, I always saw Frisk as a girl.

The question is moot, there are no bathrooms in undertale. Only doorways to hell that hold mockeries of life.