He said it's unfortunate that the US government doesn't allow sex with children or animals.

Hasn’t he admitted to changing his opinion after learning about the effects on children?

He did. The argument against him was half based on misquotes and incomplete sentences, but the other half was indeed once his opinion - he argued that age of consent was a dumb concept and that instead it should be based on what the child wants to do and any harm they were subjected to.

He later on said he regretted this view because it was explained to him that there's no ability to consent and this always causes harm to the child. His original arguments were, in typical Stallman fashion, quite obsessed with definitions themselves, almost as if the subject at hand didn't really matter he was just bothered about how the definition had some flaws.

But even with that in mind... I can't feel comfortable knowing he defended this point of view, and it does significantly harm my opinion about him.

Singular they is over 600 years old by the way

Sure, and "you" used to be exclusively plural. Language changes, and now you is exclusively singular in some parts of the world (e.g. the US "South" where "y'all" is the plural).

I want separate singular and plural pronouns. Some languages do this properly and don't even have gendered pronouns, such as Tagalog:

• he/she/singular they - siya(possessive = niya)
• plural he/she/they - sila (possessive = nila)
• singular you - iyo (possessive = niyo)
• plural you - inyo (possessive = ninyo)

there are studies around brain structure differences between men and wome

Sure, but sample sizes are small and many times they don't seem to control for hormone therapy.

That said, this one looks interesting:

A 2009 MRI study by Luders et al. found that among 24 trans women not treated with hormone therapy, regional gray matter concentrations were more similar to those of cisgender men than of cisgender women, but there was a significantly greater volume of gray matter in the right putamen compared to cisgender men. Like earlier studies, researchers concluded that transgender identity was associated with a distinct cerebral pattern.

And this one:

Rametti et al. (2011) studied 18 trans men who had not undergone hormone therapy using diffusion tensor imaging (DTI), an MRI technique which allows visualizing white matter, the structure of which is sexually dimorphic. Rametti et al. discovered that the trans men's white matter, compared to 19 cisgender gynephilic females, showed higher fractional anisotropy values in posterior part of the right SLF, the forceps minor and corticospinal tract". Compared to 24 cisgender males, they showed only lower FA values in the corticospinal tract. The white matter patterns in trans men were found to be shifted in the direction of cis men.

And this review of other studies:

A 2021 review of brain studies published in the Archives of Sexual Behavior found that "although the majority of neuroanatomical, neurophysiological, and neurometabolic features" in transgender people "resemble those of their natal sex rather than those of their experienced gender", for trans women they found feminine and demasculinized traits, and vice versa for trans men.

This suggests there may be developmental differences between trans and cis individuals, and there seems to be a correlation between trans people and the sex associated with the gender they identify as.

The body of available science certainly indicates more researchis needed and could lead to answers that show exactly what differences exist between cis and trans people. I sincerely hope that happens. But as it stands,the research isn't conclusive.

Perhaps you should believe people when they tell you who they are

Let's be extremely clear here, my support for policy will not be impacted whatsoever by the scientific research, regardless of the outcome. If you feel like hormone therapy or gender reassignment surgery or whatever other treatments will help you, you should be able to get them. Full stop. If you want to be treated as a man or a woman, I'll do that. If you want to be called by different pronouns, I'll do my best to do it, I just don't like pronouns that are both singular and plural because they can lead to confusion. That's it.

If there was a generally accepted gender-neutral set of pronouns (like in my example), I'd use them nearly exclusively. If the scientific evidence was clear cut, I'd admit as much. Neither is the case, so I'm left to find a happy medium that works for my and others in my life.

Doesn't feel like you want to have an honest argument when you ask how far we should go back on a proposal about going forward and don't address the single motivator ambiguity.