(Technically not visually X-rated, but very heavy in sexual content.)
In writing about facial expressions during gay sex (especially, during man-on-man intercourse), I’ve remarked on an ecstatic expression often shown by one partner (usually, the bottom) or both of them. From a posting on “Captioned croppings”, this example of mutual ecstasy (mouths open, eyes narrowed or fully shut):
The expressions are an outward manifestation of an inner state of mind (and body), an intense giving over of one’s self to, or losing one’s conscious self in, the sexual experience — an ecstasy or rapture. Gay men sometimes speak of a bottom in this transcendant state as being in heat; the counterpart for a top would be, I suppose, being in rut or rutting, though I’ve never heard the expression. These ways of talking adopt the vocabulary of sexual behavior in certain animals for an only very roughly analogous human phenomenon.