There's a difference between being open-minded and being so open-minded that your brains fall out. Being open-minded means considering the potential legitimacy of anything. Letting your brains fall out is where every single notion has the same potential merit in your mind, even when little about it seems to make sense. Lines do have to be drawn somewhere. Not everything can be real just because someone created a word for it.

A brain that is inappropriately masculinized or feminized contrary to a person's physical sex makes sense. A brain that changes it's level of masculinization or feminization daily doesn't. If your brain went under the necessary changes for you to flip genders on a daily basis, you would very quickly suffer intense neurological problems and die.

However, I believe in non-binary identities as a result of partial transitions meant to deal with partial dysphoria. This to me seems very real and makes a lot of sense. A brain is masculinized or feminized on a spectrum and as you begin to tip further and further to one side of that spectrum dysphoria develops. The further you tip the worse dysphoria gets. Even Harry Benjamin kind of understood this and crudely recommended non-binary/partial transitions for low-dysphoria trans people. I think this is where terms like trans-masculine and trans-feminine become a bit more accurate than trans woman and trans man.

I do also believe in people whose brains have created body maps that fall at weird places between man and woman. I believe agender people experience dysphoria, if only because they're so adamant about appearing androgynous or avoiding any gendered physical features if they can help it. I believe in trans women who like their penises. I believe in cis women who experience dysphoria over their breasts. If the body map can go egregiously wrong it can certainly go only moderately wrong.

But I'm a hell of a lot less likely to take you seriously when you make it clear to me that the label you have created for yourself is of far more importance than coping with some form of dysphoria. Being trans isn't an identity that you take on, it's a cross that you bear, and when you assume the identity without bearing the marks of suffering I'm uninclined to really take you seriously. When bi-gender people start committing suicide because science can't offer them a body that changes from male to female depending on their mood that day, late me know.