absolutely disagree with literally everything you said
one of my biggest "turn-offs" and "injustice triggers" (for lack of better terms) is lack of agency, and that includes sexual agency. saying that they can't reach their crotch is an arbitrary removal of agency, and also a generally very narrowminded and unimaginative thing to say. Who says they can't reach down there? Is that a conclusion you made just because you haven't seen depictions of them doing so? I've seen depictions that show they have that sort of flexibility, and I didn't have to look at erotica to do so. Plus...well, it's not hard to find out how horses get around the "release problem..."
secondly, making a centaur a person fused with a beast instead of its own individual, unique, manifold entity, if I may be so bold, is something no true centaur fan would ever say. In fact, anything less than a centaur being a whole, manifold entity is not only really poor treatment of centaurs and centaurs as a concept, but completely myopic. They are sapient, full stop. They are in as much control of their bodies as we are of ours. If somebody actually cared about centaurs, they'd headcanon/treat them exactly like this. Not bound by bestial nature, not subject to some ephemeral essence of "horse," not even comparable in any way to a werewolf (and even with them I'm not a fan of them losing their humanity, I just understand that's ingrained into their mythos). The centaur is itself the compelling subject, not any allegory or commentary or faux-profound exploration of the human condition - explore the human condition through the centaur, not from the centaur!
Here's where things get a bit murky, because sexual roleplay is a very ymmv thing, but I think this is objectifying centaurs a little. For one thing, it's conflating them with horses - they are not. They are centaurs. They merely look like half a human and half a horse (or one third to two thirds). Treating them like they are separate entities fused together gives me the same vibes as racial stereotypes that colonialist ignorance propagated. Sure, maybe a centaur would want to be treated that way, but in that case it's a consensual thing, not a blanket application. For another thing, I think it's actually very shocking to say a centaur is a human put into the social role of a horse. That once again robs them of their agency, and once again reads like colonialist racism. I'm not saying you're racist for thinking these things, mind you, OP, but as a fan of centaurs, and all other kinds of taurs, I want to treat them with the respect they, as sapient beings, deserve. These statements are not doing that.
Awkward and clumsy? I have one word for you: Proprioception. Yes, centaurs have a rather drastically different body plan to humans, but their vestibular system and proprioceptors would obviously be configured to account for that. For proof, we need not look any further than the humble horse! It knows how to navigate its environment effectively, and it's basically two thirds to one quarter centaur! We're literally more than halfway there already! Yes, they (horses and centaurs) aren't built to navigate our world, but they (hores, but more specifically centaurs) sure know how to navigate their own! And sapient species (and even quite a few non-sapient ones) are notorious for their capacity to adapt, and centaurs would be no different. As for humiliation, why? Why would you subject them to that? Even if they 're into it, that's not going to be indicative of the entire population. And even if they do blunder a little bit, who doesn't? Give them grace, kindness, and compassion and help them, don't degrade them for it!
But at the end of the day, who cares, centaurs aren't real. Hahahaha. No. They may not be, but that's not to say they aren't deserving of more respect and positive representation. They're a compelling creature archetype, and they deserve more than to be "person with a horse for a butt." They have a centaur butt, thank you very much.
