Cheating on one's significant other is common in Greek Myth. I have only found four gods that remain faithful: Hera, Amphitrite, Eros and Psyche (while gods that didn't marry can't exactly cheat). Examples of infidelity are: Zeus and Io.
Hades: The Most Loyal Greek God
The Greek god Hades is comparatively a better husband than his peer gods. Whilst Zeus and Poseidon – Hades' brothers – are widely known for their affairs, Hades remained loyal to Persephone. However, there are two cases in which Hades' loyalty is questioned: Minthe and Leuce.
In Ovid's famous text Metamorphosis, Hades has an affair with a young Nymph named Minthe. Persephone, now in her later years, was so incensed with jealousy that she turned Minthe into a mint plant.
Unfortunately, Zeus constantly cheats on Hera and he has done it over a hundred times, but in the end Hera always forgives him.
Ganymede (or Ganymedes) was a young man from Troy. His beauty was unparalleled, and for that reason, Zeus abducted and brought him to Olympus to serve as his cupbearer and lover. Ganymede's myth is an important step in queer history, but there is also a dark side to the story.
Despite Zeus' wandering eye, Hera remained fiercely loyal to him. While she may have had numerous opportunities to cheat on Zeus, she spent most of her time punishing the objects of his desires.
She turned him down, as Zeus had already proposed to her and by accepting his proposal she would be Queen of the Gods. Despite this, the two still held a candle for each other. Due to Zeus' constant infidelity during their marriage, Hera began a long-term, on and off affair with Hades.
In the Orphic myths, the maiden goddess Persephone was seduced by Zeus in the guise of a serpent. She bore him a son, the godling Zagreus, who, when Zeus placed him upon the throne of heaven, was attacked and dismembered by the Titanes. His heart was recovered and he was reborn through Semele as the god Dionysos.
As time went on, Persephone fell in love with Hades and they built an empire which they ruled together as equals.
Hestia in Greek Mythology
Hestia was regarded as one of the kindest and most compassionate amongst all the Gods. Perhaps the first example of a benign God or Goddess. Generally speaking, Hestia has a low key role in Greek Mythology.
In Greek mythology, Phthonus (/ˈθoʊnəs/; Ancient Greek: Φθόνος Phthónos), or sometimes Zelus, was the personification of jealousy and envy, most prominently in matters of romance. In Nonnus's Dionysiaca, he is by proxy the cause of Semele's death, having informed Hera of Zeus's affair with the princess.
Zeus was worshiped far and wide across the Greek world, including at festivals such as the Olympic Games. His legacy as the greatest of gods also meant that he became the favored deity of great leaders in the ancient world.
Like Athena and Artemis, Hestia elected to never marry and remain an eternal virgin goddess instead, forever tending to the hearth of Olympus. Despite her limited mythology, Hestia remained a very important goddess in ancient Greek society.
Sisyphus is a figure in Homer's Iliad and other works of Greek mythology. He is reputed to be the founder of the Isthmian Games and is a trickster who receives eternal punishment for trying to cheat Death.
Io, in Greek mythology, daughter of Inachus (the river god of Argos) and the Oceanid Melia. Under the name of Callithyia, Io was regarded as the first priestess of Hera, the wife of Zeus. Zeus fell in love with her and, to protect her from the wrath of Hera, changed her into a white heifer.
Zeus finally became enamored of the goddess who was to become his permanent wife — Hera.
Some estimates put the count at well over a hundred, with the lovers being mostly but not exclusively female. In some cases, ancient authorities differ, so the exact lineage and relationships remain open to debate.
She wanted Zeus all to herself, but Zeus cheated on her constantly with other goddesses and with mortal women. Hera often took out her revenge on the women who Zeus loved and the children they had with Zeus. One example of Hera's revenge is the story of the hero Heracles who was Zeus's son by the mortal woman Alcmene.
Ixion was only too happy to be a guest of the gods, but being a thoroughly bad character, he would not be on his best behaviour. The king had his heart set on wooing Hera, queen of the gods and wife of Zeus. Zeus, knowing all, pre-empted Ixion's seduction by creating a cloud which looked just like Hera.
Io. Zeus fell in love with Io and seduced her.
Suffice it to say that Zeus was constantly involved in extramarital affairs. Throughout the various and sometimes contradictory myths composed by Greek authors, there are at least 20 divine figures with whom he consorted, and about twice as many mortals.
After courting Hera to no avail, Zeus resorted to trickery by changing into a disheveled cuckoo. Hera took pity on the bird, holding it to her breast to keep it warm. Zeus then reverted to his normal form, and took advantage of Hera's surprise by raping her. Hera then married Zeus as a way to hide her shame.
Poseidon wasn't a good husband and cheated on Amphitrite with other nymphs and goddesses. On one occasion, Amphitrite got so angry that she tossed magical herbs in the nymph Scylla's bath, and the herbs turned Scylla into a horrible monster.