The bulge-nook standard is fanon; in HS proper "nook" seems to mean "ass", not vag, if anything, "bone bulge" only seems to be used as profanity rather than legit discussion of body parts, and there is no implication of tentacles of any sort. That aside, let's assume trolls do have dimorphism which would make transition meaningful; why would they be allowed to? Why would breaking that part of your assigned birth role be even semi-okay when caste roles are enforced by torture and death?

I responded to the nook/bulge thing already - though I’ll just mention this here again: 

Hussie absolutely sucked at using any of these words in any way that wasn’t insulting/joking, and also failed at using them coherently (sometimes referring to nook as interchangeable with ass or as its own thing, and even using bone bulge in one instance to mean skull). It’s clear that there’s something there, but that Hussie never thought it through beyond what he was implying - so naturally fanon came in and cleared it all up because Hussie had an entire canon he could do things with and then just did not. I think there is reason for the prehensile dick thing, but I don’t specifically remember what - and admittedly I don’t want to search through the entirety of Homestuck to figure out whether or not trolls implied they have wiggly dicks. I already did that once to figure out how frog breeding worked. It was not fun.

Onto the part about sexual dimorphism - honestly, the only way that I can finangle it is that trolls care more about castes than about gender? That gender has so little influence on their actual society compared to the strict division between blood that you can quite easily get away with transitioning pretty much no questions asked - especially since even Sollux, a lowblood, was able to transition without issue. 

If it wasn’t for Sollux, I’d probably say that it’s something only highbloods can get away with, and that they can only do so because HIC isn’t there to impose her strict rule - but then, surely highbloods would use a transitioning lowblood to enact their violence upon if it was really that much of a strict rule in their society? Why would Sollux be able to get away with the act of transitioning when even Eridan faces transphobia from his lusus for identifying as nonbinary?

This is part of the problem that comes with writing what was clearly set up to be a species without sexual dimorphism, but without making it entirely clear or deadset; there’s just things that don’t make sense even when you DO put it into a sexually dimorphised content. Alternia just isn’t built around a care for gender or for assigned sex. Its entire focus is on blood; it’s what they use to suppress lowbloods, and it’s what they use to create a natural order of things. 

Like, what importance does assigned sex have on Alternia? Well, as far as we can tell, next to nothing; though HIC is the woman in power, it’s never suggested that there’s an inherent matriarchy; it’s a fuchsiarchy. It’s because she’s fuchsia, not a woman, that makes her ruler. All of her closest in power have been men; GHB, the leader of an entire religion, and Dualscar, one of the top Orphaners in the world. So it’s not even as if we can suggest that only women can attain power on Alternia, because the truth is that they’re just as likely as the men to be stuck in specific positions based on their blood.

In terms of physical strength, too, there’s more of a bloodcaste divide than a male-female divide. Vriska outright states that highbloods are made of “stronger stuff” than lowbloods - that they can take more damage before they go down - and Equius’ physical strength, though considered a mutation, isn’t inherently because he’s male, since we know that Sollux, Tavros, and Karkat are weaker than even Gamzee. We’ve also seen Kanaya exhibit incredible feats of strength, though again, that’s not because she’s female so much as because she’s a rainbowdrinker. 

There’s not even an inherent divide on the basis of jobs; female rustbloods are just as likely to be sweeping floors as male rustbloods. And this doesn’t even go into the fact that we’ve seen nonbinary trolls in Friendsim who never really mention gendered issues - which is incredibly easy to forget, honestly - both highblood and lowblood. Like, even under Trizza’s reign - which is MUCH more brutal than Feferi’s - there’s no discrimination based on gender identity. 

So what exactly is the issue with being trans on Alternia? The fact of the matter is... there isn’t one. Everything we see of them points to bloodcaste being the biggest societal issue and the biggest biological factor used to suppress Alternians, not gender - and we’ve seen time and time again that gender identity isn’t really questioned on Alternia, either. So, even if the trolls were sexually dimorphised and could transition, they wouldn’t face anything like the experiences and transphobia humans do; it’s just not an inherent factor of their society. Hell, it might actively confuse them more than anything else. But this is completely disregarding that everything above points to a society that would put more emphasis on a gender structure based around the class system - something I’m going to go into on another post. 

Again, it’s one of those things where you can just tell that it’s a cis man’s take on society, because he genuinely couldn’t think of how society would work without a gendered structure. Well, now he’s got a gendered structure on something that doesn’t need one, and trying to figure out how trans issues have any impact on that society is almost impossible to rationalise without eventually realising that the society very likely would not care.