I think the slave must have drawn this and kept it hidden until a temporal backwash wafted it to our timeline.
Her stockinged leg obscures his groin, but there's no sign of an erection... there can't be - a permanent cage keeps his genitals clamped to his body. Perhaps he wanted to imagine what it would be like to be with her without the steel implant, but couldn't quite bring himself to draw in his penis. It simply doesn't belong.
She's wearing stockings, but carrying the whip. She wouldn't make a special performance of punishing a mere slave, so perhaps he has just serviced her so well that she now feels the need to restore the balance of power.
Many of the under-the counter slave manuals recommend just this; slaves can get ideas above their station when they've kept you whimpering for an hour, so best to beat them firmly back into their place.
In a moment, she'll whip him, then have him remove her shoes and roll down her stockings. The proximity to her hot groin will be unbearable. Her musk alone will tighten his cage until he is biting back whimpers. However, she'll dismiss him with a wave of her hand.
As she sprawls on her elegant bed, and he'll slope off to his kitchen basket, back throbbing, cock beating like a second heart inside its cage.
Perhaps sketching the lines of her thighs and ample buttocks is enough to trigger a spurt of semen. Any reduction in arousal comes as a relief, but leaves him cold, hurt, and humiliated.
Or perhaps he sketches her to preserve the glow of one-way desire just enough to warm up the bleak places in his heart?