Gender roles connect to gender dysphoria in that most people assume gender based on the role being performed (which includes gender expression e.g. wearing a dress). The reason gender roles "cause" gender dyshporia is because they cause other people to identify someone as a different gender than they identify as. The intermediate step here is gender identity, which is internal and not inherently linked to gender roles.
>transhumanists are people who get off to crsossdressing.
lmao you're thinking of transvestites
Transhumanists are people who think we should transcend our biological limitations and turn into AI superbeings or whatever.
People used to think being gay and women wanting to vote were classified as mental disorders too.
This is semantics. You could argue that someone has the "wrong brain" for their body or the "wrong body" for their brain. What matters is how it affects their life and what options there are for making their life suck as little as possible. That is the protocol for dealing with something that is a bona fide disorder, so it's bizarre that you'd use "It's a disorder" as a justification for anything else.
This is a good question, and it's hard to articulate the answer to it because we don't fully understand how it works (yet). However, we can observe the effects and it's much better to demonstrate the dynamic. You probably don't feel like you are living as the "wrong gender," right? But imagine if people started telling you that actually you are wrong and you are the opposite gender and started identifying you that way against your protests that you are actually what you say you are. Imagine that if you displayed any socially constructed traits associated with your gender identity (including pronouns or whatever) you would get people policing you for it and calling you delusional for thinking you are the gender you are.
There have been some cases of a non-trans person living as the opposite gender and experiencing similar psychological effects to trans people. There's David Reimer in the infamous case of trying to prove gender is just a social construct, but that's extremely confounded by a wide variety of sexual abuse he underwent as well. For a tamer (gender flipped) example there's Norah Vincent - a lesbian who lived as a man by choice out of curiosity. She wrote a book about it called Self-Made Man exploring how it was different from what she expected, and she ended up going to therapy because living as the "wrong gender" was actually traumatic for her.