Eraserhead

The Man in the Planet (Jack Fisk) pulls levers in his home in space, while the head of Henry Spencer (Jack Nance) floats in the sky. A giant spermatozoon-like creature emerges from Spencer's mouth, floating into the void. The Man in the Planet appears to control the creature with his levers, eventually making it fall into a pool of water.
In an industrial cityscape, Spencer walks home with his groceries. He is stopped outside his apartment by the Beautiful Girl Across the Hall (Judith Anna Roberts), who informs him that his girlfriend, Mary X (Charlotte Stewart), has invited him to dinner with her family. Spencer leaves his groceries in his apartment, which is filled with piles of dirt and dead vegetation. That night, Spencer visits X's home, conversing awkwardly with her mother (Jeanne Bates). At the dinner table, he is asked to carve a chicken that X's talkative father, Bill (Allen Joseph) has "made"; the bird writhes on the plate and gushes blood. After dinner, Spencer is cornered by X's mother, who tries to kiss him. She tells him that X has had his child and that the two must marry. X, however, is not sure if what she bore is a child.
The couple move into Spencer's one-room apartment and begin caring for the childâa swaddled bundle with an inhuman, snakelike face, resembling the spermatozoon-like creature. The infant refuses all food, crying incessantly and intolerably. The sound drives X hysterical, and she leaves Spencer and the child. Spencer attempts to care for the child, and he learns that it struggles to breathe and has developed painful sores.
Spencer begins experiencing visions, again seeing the Man in the Planet, as well as the Lady in the Radiator (Laurel Near), who sings to him as she stomps upon spermatozoon-like creatures. After a sexual encounter with the Beautiful Girl Across the Hall, Spencer has a vision where he is decapitated by a creature resembling the child, revealing a stump underneath that resembles the child's face. Soon afterwards, Spencer's head sinks into a pool of blood and falls from the sky, landing on a street below. A boy finds it, bringing it to a pencil factory to be turned into erasers.
Spencer seeks out the Beautiful Girl Across the Hall, but finds her with another man. Crushed, Spencer returns to his room, where the child is crying. He takes a pair of scissors and for the first time removes the child's swaddling. It is revealed that the child has no skin; the bandages held its internal organs together, and they spill apart after the rags are cut. The child gasps in pain, and Spencer cuts its organs with the scissors. The wounds gush a thick liquid, covering the child. The power in the room overloads; as the lights flicker on and off the child grows to huge proportions. When the lights burn out completely, the child's head is replaced by the planet. Spencer appears amidst a billowing cloud of eraser shavings. The side of the planet bursts apart, and inside, the Man in the Planet struggles with his levers, which are now emitting sparks. Spencer is embraced warmly by the Lady in the Radiator, as both white light and white noise crescendo before the film abruptly ends.

Question: With whom does Spencer had a sexual encounter?
Answer: A beautiful girl