I messaged this artist on pixiv to ask who is the person his OC Reimu has sex with in all his pictures. He told me every picture of her having sex or any sexual situatioin, is sex with her wife, who is the silver haired girl you see in his other art with his OC Reimu. He told me Reimu doesn't cheat on her wife with random men. I think they also have a daughter together, who is the little silver haired girl. Anyways, basically Reimu has a futanri wife and even though you can't see her face here and other pics, you can still tell it's supposed to be her just by the feminine features like the thighs and fingernails. I'm not going to change any tags though, since people seems really picky about stuff like this. So even though many of these pics are misleadingly tagged as as 1boy. I'll just leave it, anyone else can ask the artist themselves if they want the answer from them.