i think this is why people headcannon dave as trans

Is it? Well, to me it seemed more of a 'sarcastic cool teenager' response rather than an actual indicator of anything but hey, a reason good as any, I suppose

I agree that the quote is probably Dave being sarcastic. Personally, I always headcanon'd him as transmasc rather than transfem. I think I saw a thing about it on Tumblr back in Ye Olden Times and it just stuck in my head ever since.