I'm trying to identify a short story I read (probably in the 80s)
"The Monster", a 1970 short story by R. Chetwynd-Hayes, also the answer to the question Short Horror Story in which protagonist is a monstrous freak in hiding? on Literature Stack Exchange; see also the Sci-Fi Stack Exchange question Looking for an anthology of horror short stories from the late 1970s or early 1980s. The story was reprinted in Fantasy Tales, Spring 1991 which is available at the Internet Archive. You may have read it in the 1981 anthology 65 Great Tales of Horror edited by Mary Danby.
where the protagonist, a young woman, is captivated by the beauty of a servant, a bronzed, muscular youth working in the garden.
She had been in the new room three days when the young man appeared in the far garden. He was not the first young man she had seen, for many had passed along the street, but he was the first she had been able to study for any length of time. He erected a garden chair, then seated himself and stretched out his long legs, clearly determined to absorb as much of the sunlight as he could before moon rising. He was, she decided, a most handsome young man, and there was an
unaccountable quickening of her heart when he stripped off his tunic and bared his torso. A man minus his clothes was a phenomenon she had never considered; in fact any human being undressed had been, to that moment, an impossibility. Aunt Mabs always made her bath in the dark, maintaining it was sinful to see evenone’s own naked flesh, and Caroline jerked back from the window like a frightened bird, and took refuge on her bed.
[. . . .]
The sin in Caroline’s heart became a canker, and Satan Atomo, no doubt gloating over his conquest, made sure temptation was always before her. The young man seemed to live in the garden. He hoed the black earth that glowered
beneath the linen-white flowers, he trimmed the dark green grass with a scythe, and all the time his naked brown back screamed its untranslatable message to Caroline, so that her mind became a hothouse of searing, forbidden thoughts.
She kept away from the window for as long as she could, walked round and round the room, and prayed silently to Almighty Jehovah for forgiveness, but always, whenever her attention flagged, she came back to that fatal hole in the curtain.
The tale ends with a twist as her religious father admonishes her for taking an interest in a "mutant"
Her parents are dead; it's her uncle who admonishes her. In his view, the young man is normal; the girl herself is a mutant, whose family has kept her hidden instead of turning her in as required by law. (From our point of view, the girl is normal, the rest of humanity has mutated into monsters.)
"You thought the young man beautiful?"
"Yes, gracious Uncle, and I sinned . . ."
The Uncle raised his hand.
"You did not sin in that, for it is good that one gaze upon the beauty that Jehovah has created. Your sin is a far greater one."
Caroline trembled and waited for the enormity of her transgressions to be revealed.
"Your sin is that of ugliness. Listen to me, and I will read the holy word."
Uncle Carl opened the good book, and began to read slowly, raising his voice in the traditional style, so his words were full and sonorous.
[. . . .]
"Then the daughters of men, in their fear, took unto themselves the sons of Satan Atomo, and they did bring forth monsters.
"And Jehovah brought down the moon, and it hovered over the earth, and His voice spoke from a thunder cloud. 'Ye shall destroy all that is not born in my image.'
"And all those that still followed in the ways of the Godly obeyed this commandment. They took the ugly, the monster-born, and burnt them on the altar, and behold the smoke of the burning was good unto the Almighty, and He did
withhold His hand, and the moon fell not on to the earth, and some men lived."
Uncle Carl slammed the book closed with such force, Caroline jumped, then he spoke in the same awesome tone.
"A thousand summers have passed, and rarely are monsters born, but when they are, the people take them into the temple and burn them upon the altar."
Caroline at first did not understand, then the man's cold, expressionless stare relayed its message, and she whimpered:
"I'm not a monster."
"Child, can you not see?" The Uncle snapped out the question. "Put out your hand. Now I will lay mine alongside. Look. Mine is shaped by the Almighty, yours . . . Do not force me to describe what your eyes must see. Look at my face, then feel your own, the skin — the monstrous growth that covers your head. Can’t you understand that I feel sick in your presence, and I have gazed upon you for nigh on sixteen years. Think how that young man would react were he to see you as you are. He would hand you over to the temple elders for burning, as indeed I should have long since.
and reminds her that the perfect human created in God's image is akin to an ape. It is revealed that the story is set in the future where humanity has mutated into an animal-like form and traditional pre-apocalypse humans are regarded as mutants and an abomination in the eyes of God.
The priest raised his voice to a near scream.
"And what shall there be on the head?"
"There shall be two ears."
"And how shall the ears be shaped?"
"They will be large, and black, and shall hang down, even to the shoulders."
"And what shall be on the face?"
"There shall be two eyes, one large, one small, and two noses, and twin tusks will grow forth from the cheeks, and the lips shall be black and spread wide, and the teeth shall never be covered. Thus saith the Almighty Jehovah."
"And what shall be on the hands?"
"Two fingers, and they shall have talons, even as the feet hath two toes, and a small tail, not more than twelve inches long, shall hang from the spine, even as Jehovah has decreed."
Caroline was crying now, not because they had lighted the faggots, but because she could see her reflection in the bronze mirror. She was ugly — it was good that all this ugliness was about to be burnt. She had no lovely brown wrinkled skin; hers was obscenely white and smooth; her head, instead of being nobly domed and ridged, was covered with a grotesque mop of corn-coloured growth, which covered her horrible small ears; she only had one nose, and her eyes were both the same size, but, and this was worst of all, not delicately addled, but blue, surrounded with white, and fringed by the same hideous growth that marred her head. Each hand had four long fingers and a shorter one that stuck out at an angle, and there were five toes to each foot. Her lips were red, not black, and covered her teeth, which should have been irregular, one behind the other; hers were disgustingly white and even.