Astraea’s Atelier is an interesting story because it’s ultimately about Shu learning to disparage and dismiss art that he doesn’t personally care for on principle. This, specifically, being art he deems erotic. I think there are two ways to interpret this. In both cases, I do think that Shu has a bit of a prudish understanding of sex, but as for why that is, is up to interpretation. I personally see it as Shu is someone who is more “traditional” in terms of modesty, based on how he dresses and acts, and feels ashamed about having “human” desire so he doesn’t like to acknowledge it at all. But for people to make art about it, especially art that’s meant to arouse the viewer (like how he views Mika’s gravure lol) I think that puts him in an uncomfortable situation where he immediately wanted to reject it for being shameful. However, after the events of Astraea, he manages to open his mind a bit more to talking or viewing this sort of content. He even admits that they seemed to intrigue him to the point that he took them home to his lodging with him so he could study them more.

Another interpretation of his prudishness in Astraea would be that he’s asexual, and because he doesn’t have any interest in sexuality or erotic works, he deems them inferior and unecessary. But, at the end of the story, he comes to the conclusion that though he doesn’t understand it, it doesn’t mean it doesn’t have the right to exist. The end where Shu mentions he studied some of the artworks could be interpreted as mere curiosity of him not really getting their appeal but wanting to understand more.