Koyama tends to portray Youmu as kind of nervous around Reisen, at least that's what I've gathered from it. I kind of like to think its because Youmu would be a little embarrassed doing it in say stationary, she would die from Koyama's Reisen's cuteness. (this is just IMO, cause a lot of artist tend to do that with a futa Eiki and Komachi as well Yumiya is a prime example here post #693242)
However, I do recall seeing an image of these 2 doing it, with Reisen on top. But, yeah, I do have to agree with you I'm not compliainig either. :)
Man, Koyama Shigeru deleted all his futanari works from pixiv. Why would he do that?
Because he wasn't proud of them anymore? Because he wanted to move on? I can identify with not wanting people to see certain examples of one's content, if that is indeed the reason.