A few years I stumbled upon a manga page on an /a/ thread depicting the act described in this wiki article.
https://en.m.wikipedia.org/wiki/Osculum_infame
The demon looked muscular and didn't look too weird, basically tall humanoid with horns and bat wings. The witch was naked pretty sure, and I remember her being drawn in a kinda mature way and with a large chest. Anyone by any chance know what manga it could possibly be? Pretty sure it wasn't an actual ecchi manga.