I see absolutely nothing in the picture that could be badly interpreted, even for someone who doesn't know anything about the story. That's just lovely art, no context needed
^^ I was about to agree, but then I realized 'Oh it was made in 2013', that’s just how anime faces showing shock were drawn back then it seems, haha :D.
From the looks of it, Aya and Mitsuki spent the night together, and it clearly seems to be based on the latest chapter, where they’re adults now. Mitsuki has new tattoos, one around her left arm and another on her right forearm, and Aya looks like she’s snoozing her alarm, so I don’t really see what’s wrong with the photo.
I noticed this series has pretty significant JP fanbase. Which makes sense considering how the aesthetic of the show can easily be passed as a live action anime adaptation because of how eccentric everything is.