There's too much confusion in the air
People are mixing up ethics, ethics guidelines for profession, and morals. Morals is just the social consensus on what is appropriate behaviour. It's definitionally relative, because societies reach a consensus (or don't) and this process has nothing to do with right and wrong. Ethics guidelines for professionals or research ethics is not ethics either. It's an objective standard of behaviours and principles. Now, there is an ethical obligation to do this, as well as practical and a morality-based argument for doing this.
And, finally, ethics is the branch of philosophy that examines the questions of right and wrong. It does claim universality. The concrete guidelines for ethical behaviour naturally change as we become more aware of the facts on the ground, such as when we realise that putting bleach in a rectum is bad for you. However, the ethical principle telling us what to do did not change, merely our understanding of the act did. This is not to say that ethics does not progress -- of course it does!
This is wikipedia level stuff, not sure why the discussion went on as if the concepts used by the parties were the same when they weren't.
It becomes unethical when you're forcing and/or peddling the treatment on others, particularly children, and especially for a profit.
If you want to pour gasoline up your own ass for shits and giggles, by all means, have at it. You're fucking dumb as goddamn shit if you do it, but hey, there are no laws against stupidity.
The second you try doing that to your kids, however, is where you've crossed the line.
Some have argued from Locke that one has the moral right -- one isn't acting unethically -- to stuff bleach in one's own rectum. This is, ofc, not the case, based on Locke's TTG 2 VII. From utilitarianism, Kant, and Rawls it's obvious that we don't have that moral right -- we are ethically in the wrong to sef-harm.
Why is this? For and from Locke, estate and liberty are only relevant if you have the health and life to exercise your liberty and enjoy your property. Now, many American philosophers like to downplay this because of their politics, so it is not surprising that you don't hear this in an American university.
For Kant, it's obvious that it cannot be the universal right to harm yourself. From the utilitarian position, the society as a whole is worse off if people self-harm. For Rawls, it is a question of fairness: since a Rawlsian society would have publicly funded health care, self-harming leads to use of resources that ought to have been used for the treatment of injuries that were not sustained by willful stupidity.
In the special case of a parent self-harming, there's just pretty much no justification under those ethical theories for calling it ethically acceptable, and Aristotelian or virtue ethics can be v easily added. (How could we justify self-harming as something a good parent does? A good parent would obviously choose whatever option it is that ensures the best outcome for their child)