As a female sexual demon, a succubus, the creature would beguile a man into intercourse and so obtain the man's seed. (The word succubus derives from the Latin 'to lie under'.)
Then, appearing as a handsome male spirit, an incubus, the demon would make love to a woman and spread the seed in her. (The word incubus comes from the Latin verb 'to lie upon'.) In medieval times, the wizard Merlin was believed to have been born as a result of a demon-woman mating.
Taking these ideas, I made the incubus in my Dark Maiden deliberately androgynous - eerily beautiful but possibly of either gender.
Here's a brief excerpt:
Somewhere during their kissing her anger vanished.
From inside the hut she heard a broken sobbing. Father William, she hoped, finally poleaxed with remorse.The rowans shook with a sudden wind and the rooks cawed. She kissed Geraint again. Sensing the chill air trembling around them, she turned.
A sour-faced, beautiful being, neither male nor female, appeared immediately in front of them in the clearing beside the priest’s house.
“I cannot stand against you both.” With this complaint, the incubus scowled and pouted, like a young virgin of either sex. The winter light shimmered on the demon’s flawless skin, lit hair that at times looked golden, at times black and revealed a lissome body clothed in a white robe. Or was the long, sweeping tunic red?
(The picture of Lilith is from a painting by John Collier.)