oh man.....thats pretty artistic can i add the"sex"tag cause its pretty obvious whats going on here... at least it seems shes enjoying it...... ;-;
The image is not showing an explicit image of her having sex, just a shadow implying so, so it doesn't warrant the sex tag, implied_sex would be more likely. Remember, tag what you see, not what you know.
You should really check what led to this in his book. "Masho no Ushio Zoku" I recall.
Yeah I saw it(even if I didnt like/get the doujin), but Im just talking about tagging purposes only, this being implied sex would imply that every girl undressing here would end the same and tag that.