This is actually a topic we discussed very early on in development. Given everything Asterion has been put through, was he ever sexually abused? The Ruthless route gives a glimpse at the sort of stuff that can happen, and it's not pretty. By the sheer horror of what goes there (or by a Borgian mindset that given enough time everything that can happen will happen), one can certainly deem it "plausible."
As you put so well, it's an uncomfortable topic. It's a spiky question that can make it hard to enjoy some of the sweeter moments in the story. The doubt is bitter, isn't it?
Now, we are very deliberate with the things we say and how we say them... and we are equally deliberate with what we don't say. And the same goes for Asterion, who has thousands of years of experiences and only so much time, so many opportunities, to say things. And he knows that what he reveals about himself changes the way people see him. In fact, the mere choice to tell the MC about how the hotel works is world-defining, as it sets in motion so many of the things you can do in the story.
In fact, maybe it's worth stopping to think for a second just how much of what happens only happens because Asterion tells you things at the right time. Maybe people should reflect about what it means for Asterion to have created the hotel himself, then made it so the human master gets to be under the spotlight while he's the one defining its future across generations.
With that said, I will not answer the fundamental question of "Was Asterion ever sexually abused?" Whatever is the truth, whether there was any sort of abuse or not, to give an answer would make it too easy. Instead, as a reward for being so inquisitive, I will give you broader questions as a prize: what is Asterion not telling you? What goes on inside his head which you are not privy to? Sometimes you'll have a brief chance to glimpse what's on his mind, but just how much did you see? What is the mind of a man who's been through so much for thousands of years like? Can any person in a single lifetime learn all there is to know about a man like this?
And, to top it off... By my tone here, just by my acknowledging that this was a question we thought about early in development, someone could make the assumption that obviously it means that the answer is "yes, he was abused." But I'm not letting this hypothetical reader off the hook so easily. In truth, when you keep in mind all the questions I posed in the previous paragraph, I want you to be aware that the truth may just as well be "no, he wasn't abused" and we still have yet darker things in store. Maybe we considered sexual abuse too easy a topic. Or maybe we covered it elsewhere and you didn't notice.
You know, when you stop and think about it, the valley truly is a horrific place. And, worse yet, maybe these truly horrific things are already in the game, and you never noticed them! This goes down to the very way in which the world of Minotaur Hotel works — one can perform fully functional, world-changing rituals like Pedro and Oscar do without being even aware that they are doing it. Perhaps you, as you played the game and picked your decisions, performed rites without knowing — and just what effect did they have?
But is the valley even the darkest place in Minotaur Hotel?
I hope this message of mine did not answer your question. In fact it is my hope that it was both rewarding and uncomfortable for you to read it. However, I will congratulate for playing the game inquisitively, which is one of the meatier ways of playing it.
As a final rubbing of salt on the wound, though, I will say I deleted a few of the paragraphs I wrote for this post. They said too much, knowing all of that stuff would have made it too easy for you.