Frankenstein monster is kinda like Trans folks or people

[Sheldon]

No human method can be infallible of course, and the reliability of branches of science vary of course, but this does not mean that they are entirely subjective. Thus science's insistence on keeping an open mind, and that all facts must remain tentative in the light of new evidence, and its ability to admit of an error, and rethink their position in the light of new evidence, is in fact one the methods greatest strengths. 

Knowing something was wrong  or incorrect, still advances knowledge. Clinging to unfalsifiable or demonstrably incorrect doctrine and dogma has the opposite effect. 

Lets remember your claim was that all we had was the subjective opinion of the patient, this clearly is not the case. This is also a complex issue, as we are not just dealing with objective facts, but subjective moral judgments.

Well clearly if there are objective markers for identifying gender dysphoria, this demonstrates that gender a) does not always reflect biological sex, and b) that we are not limited to entirely subjective claims about gender, as you claimed.

See Paleo's response above.

He says that the symptoms of the dysphoria are caused by cultural conditions, not by the person's gender. 

They're not mutually exclusive, as was explained earlier, since gender is primarily a social construct, so it can reflect biological gender, or not, when it does not, then we see gender dysphoria. The addition of bigotry, based on cultural stereotypes obviously does not help. 

So I don't think that the presence of gender dysphoria can be taken as objective evidence that a person is one gender or another. 

I don't understand what you think "is one gender or another" means? Gender identity and biological sex, are not always the same, gender dysphoria occurs when a person's gender differs from their biological sex, this can be objectively evaluated beyond the purely subjective claim. 

Only that they feel a certain way about their gender, 

I doubt this is how psychiatrists evaluate gender identity, and the resultant gender dysphoria. Why would we assume this is just a feeling, we don't do that when people claim to be gay? Well some people do obviously, the old "they're not born that way, it's a lifestyle choice" nonsense. 

"I feel that I have a body of the wrong gender" is subjective testimony.

On its's own, but then so is the bare claim someone is gay, and we have already shown that psychiatry can evaluate the claim with objective markers. 

What they used to call a disorder is simply the stress caused by society's reactions to that testimony. 

Using that rationale one could claim people aren't really gay, they're just reacting badly to negative feedback about they feel their sexual orinetation is? I have to say I am dubious that it is that simple.