I support radical bodily autonomy. I support people's right to have full control over their own reproductive system. I support trans people's right to live in peace, comfort, and safety. I support universal, free, and effective healthcare. I support disabled people and their right to live in a society that works proactively to mitigate the impact of their disabilities.
Transhumanism is a rejection of the naturalist fallacy. No one needs to suffer. We can, and should, work to fix these horrible broken bodies we are shackled to. People should be free to do whatever they want with their bodies, and we should work towards increasing the control people have over their bodies as much as possible. We can, and should, find ways to mitigate human suffering.
A lot of transhumanists are cranks, eugenicists, libertarian freaks, effective altruist, or barely cryptofascists, etc. Fuck 'em.
A lot of them are obsessed with hypothetical technologies - Magical nanotech that totally ignores entropy, obsession with magic AI gods that will take them to secular heaven, digitizing their brains so they can be truly terminally online. That's all bullshit.
What matters is practical technologies and philosophies that are being developed and implemented right now. There are nerve stimulating implants that help people with spinal injuries poop better. There are nerve stimulating implants that help people with apnea sleep through the night. Biomedicine keeps kicking out drugs that mitigate a wide variety of illnesses and diseases, from heart problems to depression to mitigating some of the most horrific effects of aging.
Refusing to help people live better lives because 20th century fascists engaged in medical torture in an attempt to manufacture an ascientific "master race" is cowardice. People with endometriosis deserve a cure. People with bipolar disorder deserve a cure. People with nerve damage deserve to regain the full use of their entire nervous system. Amputees deserve a biologically cloned limb fully integrated in to their nervous system with no loss of motor control or sensation. People deserve to live as long as they want to without suffering the horrific, degrading effects of aging.
Communism isn't enough. A better economy isn't going to mitigate or relieve the symptoms of schizophrenia or IBS. It's not being to give visually impaired people the option to have their eyes or ocular nerve repaired or replaced. It's not going to repair traumatic brain injuries. Fixing these problems requires a radical rejection of the notion of a "natural" human and the acceptance of the human body as a machine that can be changed and manipulated to suit the needs of it's user.
And frankly, whatever you think of it, or however you conceptualize it, it's already here. Radical manipulation of the body is normal and widespread, whether it's replacing joints with mechanical components or installing pacemakers. Cloned organs are here - cloned skin is being used for skin grafts. We can connect cameras directly to the optic nerve and control computer cursors with brain implants. And the first genetically modified humans have already been born.. Pandora's bag is open.
The question around transhumanism isn't whether or not it will happen; It already has. The question is which ideologies will control the process, and whether it will be used to improve the human condition or enslave humanity in horrifying ways scarcely imaginable.
If you want to see a leftist exploration of a transhumanist future, dealing in depth with both the potential benefits and the potential horrors, you should go read the background materials for Eclipse Phase. (it's made by anarchists and released under Creative Commons. You can download the whole system online) Eclipse Phase is a TTRPG setting built entirely around a core of a transhumanist future where human minds can be uploaded as information, swapping bodies is essential to the function of the economy, genetic engineering is universal, and there is a solar-system wide cold war between leftists trying to democratize transhumanist technologies so everyone benefits from them and hyper-capitalists who want dictatorial control so they can rule over humanity forever. And it's explicitly a cosmic horror story - The setting does not pull any punches at all about how horrific this future will be if it's left in the hands of capitalists and fascists.
This is intensely personal to me. I have Bipolar disorder. It's a highly heritable, ie genetic, condition. Myself and many members of my family have suffered horribly for generations under the burden of this and related illnesses. I can't have children because there's a very high chance they would inherit Bipolar disorder. Bipolar people have an unemployment rate of 60%. 80% of our marriages fail. Our suicide rate is horrific. And even those of us who don't kill themselves have their life expectancy slashed by a decade as a result of the constant chronic stress this condition inflicts.
I want a cure. I want someone to dig around in my DNA and build some kind of magic bullshit nano-machine that will edit it out of my genome. I want to know what it's like to live life without the constant miserable agony of depression, cognitive impairment, chronic stress, and trauma. I want a choice about whether or not to have children, and I want to be able to make that choice knowing that my children would not be condemned to a life of misery and suffering due to the horror written in to my genes.
Rejecting a better future because you're afraid of the specter of Eugenics is cowardice. You've already accepted that the same mechanical technology can be oppressive or liberatory depending on whether society is run by capitalists or leftists. You need to accept that biotechnology asks the same question - Will we control it? or will the enemy?