God I can't believe this is what lead me to write in the forums, hah!
I will preface with this: This is your story, and I get, and it is ok if you want Asterion to be a bottom, if you want to write him that way, it IS your character and you can do with him whatever you want. It is fine, and even though I would like him to top some more than what you allude he will do, It'd be only a minor disappointment, really. Now, however, I want make a correction on a misunderstanding you seem to have, and I am gonna blame it on you (you guys? plural? can't remember if it's more than one writer or not.) being Latino (I THINK I heard you were?). Us latinos have the word for top being "activo/ativo" and bottom be "pasivo". This implies that most if not all tops are dominant and most if not all bottoms are submissive. This is kind of ingrained, and even I have to de-learn it, but this is wrong. (Even if you're not latinos tho, some people even in the gay community still think that bottom means submissive too...) And I will argue that, honestly? If you're writing the character to make sense for the story you want to tell, being a bottom from the get go makes actually little sense (And I say this, as a DOMINANT, switch leaning bottom, so even though it sounds like I might be biased, I'm honestly looking at it through a narrative perspective)
First things first, Bottom. Bottom just means the guy who gets penetrated, it has no bearing on if a guy is dominant or submissive, though it's viewed by a lot that way, but that's far from true for a lot of people. There are submissive bottoms, and there are ALSO submissive tops, guys who don't like to be fucked, but only fuck, but at the same time like it when the bottom takes charge, leads them, teach them, order them around, what have you (another brand is Lazy tops, those who just like to be ridden cause they're lazy and can't be bothered to put the effort, but Asterion is no lazy boy no sir). On the same vein, there are dominant bottoms, who like to take charge, boss the top around, straight up "ride the bull" as someone already stated.
So where I go with this is... Look, I get it, actually. I follow your logic, really, you just started from the wrong premise and got to a weird/wrong conclusion. If you were to replace "bottom" with "submissive", then what you wrote makes absolutely perfect sense. He does not have the self confidence to be *dominant* in bed. Him taking the charge, as a dominant would, during his first sex experiences would be jarring and out of character for him. It would make more sense if the MC actually leads him in, teaches him the ropes, specially if the first one or first few is just about exploring each other bodies, and no penetration is involved. Learning about what makes the other tick, etc, with the MC taking control, leading the way, and when Asterion has gotten his confidence all the way up, then and ONLY then is he comfortable to maybe take charge and be dominant. None of this alludes to his sexual preference on giving or receiving anal. If we're going for cohesiveness, I honestly believe this would work better, but of course I'm not trying to write the story you're telling, it is you who write it, I just want to fix a misconception about sexual preferences that kinda would go against the story you want to tell.
My point here is... Asterion's sexual awakening and story, if it's going to be told in a consistent and cohesive way, should be a submissive guy who does not know what he wants yet! He can end being a submissive switch who prefers to bottom and can get to be dominant from time to time, which seems to be the implication of what you want to write both for his character and fan-service alike, but he should not start that way! You have written him in very consistent ways, and he keeps stating over and over that he doesn't know what he wants, refuses to acknowledge his feelings, feels ashamed of them, does his best to try not to think about anything sexual, dare he to dream of this? And he has never in all his millennia alive, ever considered sex or a loving partner, because he thinks himself a monster. And, if Storm is his supposed parallel, he probably never even figured out if he was gay or not until he caught feelings for the MC! This would imply he never, ever had considered what he would want to do with a partner, not until now.
Now my third point is a touchy subject, and that is sexual abuse... It hasn't been stated if he's endured that, but it is not unheard of for slaves to be abused and sodomised with objects, and if the purpose of a jailor was to deliberately make him suffer, there's nothing stopping them from giving him further humiliation this way. This would mean that Asterion would be more reluctant to bottom, even if he trusts the MC, than the other way around (and there is some... beauty in the ide of guiding him around learning what he likes, letting him experiment with other stuff, letting him top first, then finally him trusting MC to bottom, and finding out he LOVES it, and ending up being a mostly bottom guy, now this would definitely be in line with the story I think). I don't know if sexual abuse will be part of Asterion's past, and I honestly don't want it to be because my poor baby moo has suffered enough, but it would fall in line with him suffering through every humiliation and punishment under the sun... This is why I would say, him being a bottom from the get go would actually be a more Jarring and out-of-character scenario than him being a top, switch, or more fittingly, not knowing which he is.
Lastly I do have to point out something that would work on your favor on him being a bottom, and that's ancient greek/cretan/roman believes of sexual position and power dynamics, and he might have those ingrained in him, but I would argue (though this is speculation and you have the last say on that), is that you wrote yourself into a corner with his story if we're gonna go that route. First, he was a very sheltered moo who, up to age 12 I believe, when he was sent to the labyrinth (this would be before or JUST around his sexual awakening, so even if people told him about sex and such and power dynamics I doubt he would have understood or taken anything like that to heart), took a while to have human interaction again, and that was with just one guy, for 3-4 years, and they were more interested in trying to convince him to kill their dad, then on physically training him, than having sex with him or teaching him "yo never bottom that's gay, dude." Then, of course, is 3 more years in the Asphodel Meadows, where supposedly the souls there don't suffer, and are all equal, so why bother with power dynamics and such? (though I don't even know if they have sex in there, or care for it? but this is up to you of course, tis your story) But it's stated Asterion is thought to be a beast by everyone, whether they're friendly to him or not, and would not try to romance him or have sex with him, so (as far as I know) he wouldn't even try to consider thinking about it nor people would teach him anything of this. His next human interaction were his jailors, who would just torture him in any way they thought, so it isn't until the hotel is created and guests arrive that he may have found some knowledge about same sex, power dynamics, what have you, but the thing is, he wasn't just getting guests from greece, rome, etc, he's supposed to have gotten guests from all kinds of cultures, and there are cultures where gay sex was considered ok, normal, it was fine to be either top or bottom, there are some Native American tribes who thought gays were a "third sex" and they had partners just the same and it wasn't about being bottom =submissive, so I personally find it hard to believe he would really have it ingrained that if he's submissive he must be the bottom, though it is true that he was locked up during the gay liberation era where it TRULY became ok in the eyes of most of the world... however, even if you write it as he does have these believes in him, it is what he may think he wants, because he's "not dominant", but the MC knows better. He SHOULD know better, and even may have a preference (or not, switch life!) whether he's a seasoned guy or a virgin (from the options given, either is possible), from a top to a bottom, he does have the internet and porn, so he should know what he wants or wants to try, and if Asterion doesn't really understand himself, it'd be up to the MC to either teach him the ropes, or both to learn together what they like or not. This, I believe, would be a more fitting scenario.
Now, as I stated at the beginning, in the end it is your story, you are the one who will write whatever you will see fitting for your character, your story, and everything. If you decide to have him be a super bottom from the get go, I will just have to swallow my copium pills and that's that, I will not stop enjoying this Visual novel whatsoever, just think it's a bit weird how sexually Asterion would be written. I have said all this from the bottom of my heart, I truly hope I didn't sound condescending or annoying in any way, I just think that, if you were writing Asterion to be this way because of story cohesiveness, to fit story themes, etc, this would be a better way to go. If you do care for story cohesiveness (And after reading and re-reading this VN for 4 days almost straight, finding new nuggets of lore every time and seeing how much attention to detail is put to every little thing in this story I literally have no doubt that you do care) I just hope you will at least consider what I said.
Thank you for coming to my TED talk.
TL;DR: Asterion should be a submissive who doesn't know what he wants, actually.
P.S.: By the Twelve I am soooo sorry for making you read (if you did) essentially a fucking 3 page dissertation on Asterion's sexual preferences, as I said I say this from a position of love, I have never read a novel that hits me as hard as this one did. I consider Minotaur hotel an actual masterpiece. It's been years since any piece of media had me be forced to just stop for a few minutes due to inconsolable, loud sobbing and ugly crying (last time was after watching anohana, if you know... You know), and It wasn't just once but constantly! The attention to details, how deep the lore is, how deep the characters are (and hot, of course), It's just pure perfection, and although I do have to admit I'm the ever so slightly biased for wanting bull cock, It is not the main nor the biggest reason I wrote all this. In the end though, I'll accept whatever you decide to write for my little pogchamp star, as he's already perfection upon this earth.