This is one of the things that makes me wonder whether people with ASD are overrepresented in GC feminism just as they are in TRA and aligned groups (though perhaps not to the same extent).
I have an ASD diagnosis, and think it's related to the fact that while I can lie, I feel far more uncomfortable with doing so than my understanding of how the average person feels about it. It's impossible to truly know how everyone else feels, of course, but the impression I get is that it's normal to be reasonably comfortable speaking untruth if it would make things go more smoothly and wouldn't harm anyone. I didn't lie at all as a kid (made my parents so proud at the time… they didn't realise at the time that it wasn't just their excellent parenting but was actually deeply developmentally inappropriate 😂), and when I did learn to, it would turn out to be at the cost of quite strong mental discomfort. And I tend to differentiate types of deception along slightly different dividing lines to the ones people generally seem to use — there are things I'd think of as just plain lying which others use nicer words for.
Words coming from my mouth which don't match what I know reality to be… it makes me feel like I've accidentally walked naked into a roomful of people. Exposed, like everyone can see that what I've said and what's in my head aren't the same. Or… something like shame. There's a lurching feeling. It's hard to explain. But using "she" for someone I know to be male or "he" for someone I know to be female takes a conscious effort every single time I have to use one of the relevant words, with feelings of self-consciousness and exposure and guilt, sometimes layered with resentful feelings towards the people who expect this difficult task of me for the mental ease of others, and embarrassment that I'm submitting to an irrational social demand to obtain the approval of others.
It's not like a lot of other kinds of requests people make to change one's speech to save others' feelings — if you're a communist or a misogynist or deeply religious or an anti-vaccine campaigner or a gay rights activist or an anti-Semite or a vegan or an anti-poverty campaigner or an atheist or think gay marriage is an abomination or like to swear a lot, or any number of other things that mean that some things you might want to say could hurt some other people's feelings, you can choose to not say the potentially upsetting things. I've certainly chosen not to mention that, say, I don't believe in God, when someone's sharing something like the fact they get comfort from praying after their bereavement. But you're generally not required to actively make statements that are the opposite of what you believe. With gender, seemingly there is a social requirement to lie (if you don't genuinely think inside your head of that person as matching that pronoun), and for me it's a very difficult and uncomfortable task, one that I think is unfair to blithely demand of me as though it's nothing.
I think my difficulty with lying, and with telling when other people are lying, is also connected to my low tolerance for deception from others. I'm a reasonably forgiving person, as far as I can tell from my own subjective perspective, but discovering someone's lied to me is an alarm bell. When you struggle to detect deception, it's important to know you can trust the people you choose to spend time with. So I have a strong instinct to avoid people I know to be liars, and when someone is claiming to be something which I know they're not, it's hard work to turn off that alarm.