the_dunk_tank

beastiality is not at all permissible because they have no desire to do anything like that with human beings.

See, the author's article gives examples of an animal eagerly mounting a human. You have probably had a dog try to hump your foot or a stray cat in heat brush up against you and lift her tail. They certainly express this desire. Your formulation does not allow us to forbid bestiality in such cases, which is a serious problem. We must instead explain away the desire via interpretation, which is not trivial: animals masturbate and do all sorts of things in nature, so we can't say their true goal is procreation or even sex with others animals of the same species. It becomes an exercise in motivated reasoning.

Similar objections apply for humans, since there is no objective way for an external party to tell the difference between "what they actually want" and "a fleeting impulse". I think it is far better to use a standard that is separate from any individual. Courts routinely compare actions against those of a hypothetical "reasonable person", and you can have a fiduciary duty to act rationally in another entity's best interests. It's possible to agree on what these legal fictions require. Your test is doubly unknowable. It asks us to first to know the true desires of the party in question, which is an interpretive task. We then must apply that desire to the definitionally-incomprehensible-to-the-party decision at hand, which is also interpretive. If we interpret the pig as having a desire to be free, does that mean that it really wants to go on some kind of hunger strike, or put its babies out of their misery? Totally abstract and impossible to agree on.

I suppose you could argue that I’m just rephrasing your point, but I think it’s an important way to rephrase it, because the logic of “best self” could be used to override eg kid’s consent to HRT and tell them they can’t get it, despite them wanting to, because as a Christian you think they just lack impulse control and need to be disciplined.

Yeah like you say, I think the way you rephrase this idea is ultimately not different. The bad decisions now come through the interpretive power given to the guardian, who can say e.g. "my kid is showing a desire to live a healthy life, and because they're a kid they simply do not understand that HRT will hamper that goal, therefore I'm gonna withhold HRT". It's a terrible thing to be responsible for another: if they are granted any power at all to override immediate consent, guardians will always be able to mistakenly make bad decisions on behalf of their wards. But I think they can at least be given responsibilities that people can reasonably agree on.