What simile describes how Mary feels in his company?

A simile is a figure of speech in which a comparison is made between one thing and another, using the words "like" or "as." Examples include "As brave as a lion" and "As quiet as a mouse." In fact, the title of Dahl's short story is itself a shortened version of a simile: "Like a lamb to the slaughter."
In "Lamb To The Slaughter," Mary Maloney initially gives the impression of being a very devoted wife. Her role is a traditional one, that of mother and homemaker. She is very submissive to her husband, always going out of her way to cater to his every need. There is no sense, though, that her devotion is in any way less than genuine. This is reflected in Dahl's use of a simile to describe how Mary feels toward her husband:

She loved to luxuriate in the presence of this man, and to feel—almost as a sunbather feels the sun—that warm male glow that came out of him to her when they were alone together. 

Mary is not simply devoted to her husband, she positively idolizes him. He is the center of her solar system, the sun around which she orbits. The simile also illustrates Mary's timidity and submissiveness. She sees herself as a lesser planet in relation to the huge ball of fire and light in whose bright rays she warmly basks. The simile is well chosen; it perfectly encapsulates the central relationship as it stands at the beginning of the story. (This is not the case, of course, at the end.)