Demeter's immortal lovers were Poseidon & Zeus, her mortal lovers were Jasion, Karmanor and Mekon.
Demeter is both sister and lover to Poseidon, together they had Despoine, a mystery goddess; and Areion, a immortal horse.
Yes Poseidon is the brother and lover of Demeter, their parents are Rhea and Cronus and thier children are Despoine, a mystery goddess and Areion, a immortal horse.
In Greek mythology, Zeus and Demeter were not lovers. Zeus was married to Hera, and Demeter was known as the goddess of agriculture and fertility. Zeus did have many affairs and relationships with other goddesses and mortal women, but not with Demeter.
Demeter did not have a husband, but she was the lover of Zeus before he married Hera. She does have a daughter, though; Persephone, (minor) goddess of nature, was taken to the underworld by Hades and now lives there half a year. She is not married.
Before Persephone became the wife of Hades, he had a lover called Mintha and either Demeter or Persephone turned her into the herb, mint.
With Jason of the Argos, no. The lover and consort of Demeter was named Iasion also called Iasius, was, according to some, a son of Zeus and Electra, the daughter of Atlas, and a brother of Dardanus; but others called him a son of Corythus and Electra, of Zeus and the nymph Hemera, or of Ilithyius, or of Minos and the nymph Pyronia. He met and loved Demeter at the wedding of Cadmus and Harmonia.
They are most often associated with a myth, such as a screech owl was what the gardener of Hades, Ascalaphus, was turned into that bird for telling that Persephone had eaten food from the Underworld by Demeter. A white poplar was sacred for being the metamorphosed form of Leuke, a lover of Hades. Minthe was of that same origion as a lover of Hades who was transformed by Demeter or Persephone into mint. Most symbols are then crude memory cues for the myth involving certain gods and goddesses.
There was no "god of Demeter", Demeter was a Greek goddess.
He was the son of Cronus and Rhea, the brother of Hestia, Hera, Demeter, Poseidon and Hades. Also the husband of his sister Hera and the lover of innumerable mortal women.
Demeter
Demeter was not married. Demeter had two consorts in Karmanor and Iasion.