When I did my first playthrough on the previous version of the game I admit I was mostly waiting around for some Minotaur action since I generally don't play VNs.
After my playthrough on the current version I have to say I enjoy the story and characters enough that I'm fine with what we've gotten so far in that department, I have enough of an affection for Asterion as a character that I want to see the romantic aspect of the story develop naturally. I think the writers have done a good job with this and I like that it's a believable relationship and not some generic 'You were nice to me three times master and now I love you forever'
Whether he's a top or a bottom isn't really important to me, I think him being either could work with enough time and context, and I don't think his submissive nature is reflective of either, it's more likely the result of him being secluded from people and kept in servitude for most of his life, and being wary of the Master. He gets to be fairly assertive about what he wants later on in the story.