He knows it's a sin to touch such bright skin. After all, isn't that who he is, a Sin of his own, a homunculus embodying humanity's sin of Envy? The skin before him, the golden hue of one used to the sun gently tanning him trembles and heaves under his unskilled, hateful touch. Each touch tries to convey his thoughts, his hatred, his envy.
The child of that woman. That slut of a woman, the woman that welcomed Hohenheim into her arms, her house, her body as he ran from a woman and dead son that still had a spark of love for him. Several times, as the body before him, uttering curses and unable to push him away, was simply the eldest of her children. He suckles a little at the flesh arching into his mouth, covered by errant strands of sweaty golden hair.
He knows this is the first sexual encounter this child, a child of only teen years, has had, perhaps even the first sexual stirrings. Watching the child over his lifetime, it's almost enough to believe he has more interest in alchemy than sex. It's almost asexual, laughably asexual as he pulls the naked body against his roughly clothed one.
This golden child was a favorite, favored of the military, of civilians, of the rest of the homunculus, even favored between all the sons and the father. Even of his hatred, he was favored to be the one to receive Envy. A rough bite marks his sin, the lustful act barely tinged with something even so simple as sexual need. One touch, one way or the other, would push the experience into land neither could escape from.
It is a sin, to pull forth unforgettable pleasure that will be unattainable from that moment on. A sin, to mar that innocence of sex, to put physical feelings with intellectual knowledge.
A sin, only completed by a thick white liquid as his body bucks in pleasure brought on by hatred and Envy.