The Ancient Greeks were not as prudish and puritanical about fidelity as many Americans are today. Americans all to often romanticize marriage with idealistic notions of fate, destiny, and lifelong commitment; others see it as a moralistic institution because of Judeo-Christian codes of sanctity.
The Greeks were obviously not from either of these ideologies: they were humanists who saw humans as individualistic and rational creatures. Even their gods suffered from human vices. Zeus was not monogamous with Hera: he fathered countless other gods, goddesses, and demi-gods.
Thus, is the plight of Odysseus, the epic hero of The Odyssey. To us, his relations with the goddesses make him look like an adulterer, and his belief that Penelope be faithful looks like a double standard.
Homer must make Odysseus epic: in battle and in the bedroom. He must have his virtues and his vices, such is the duality of Homer's humanistic portrayal. Like Zeus' infidelities and Poseidon's wrath, Odysseus too has a weaknesses: vanity and hubris. Because he is the champion of Troy, heads above his mortal colleagues, Odysseus feels deserving of the good things in life: wine, women, and song (much like Tiger Woods, who admitted to such over the weekend in his public statement).
Odysseus cannot resist a goddess. Going to be with Calypso and Circe gives him immortality, and it shows his greatnesses. He is desired by the gods. Even Athena wants this of Odysseus: she condones it.
After a while, Odysseus grows bored of the goddesses, such is his desire to rejoin the human world. This is the basis of Greek humanism: human qualities are more important than supernatural ones. So, Odysseus' longing for Penelope, who is older and less attractive than the sea nymphs, is the strongest form of love, according to the Greeks. Odysseus rejects immortality in favor of home and wife.
In this way, Homer makes his hero immortal. In the end, Odysseus is human, like us, great in his sins and suffering.