In the opening scene of Medea, the audience is informed of the conflict that is at the heart of the play.
The narrative is delivered by the woman who serving as wet-nurse to Jason and Medea's children, she tells us that Jason has betrayed his wife and offspring, by seeking the bed of a royal bride.
The background is this:
Upon arriving in Corinth with his family, Jason is a man without a country, an exile. He had left his own household to live with the King, Creon, and his daughter the princess Glauce.
Jason is a famous man, and a celebrated hero after succeeding in his quest to acquire the Golden Fleece. Creon was happy to have him at court, it enhanced his prestige.
Jason and Creon developed a solid relationship, and after a short time Creon offers Jason the hand of the Glauce in marriage.
Jason, of course, is already married, to Medea, who is herself a royal princess, though like Jason she is an exile, she is also a foreigner and she has no standing.
Glauce is Creon’s only heir, and if Jason were to marry her, he would then inherit the kingdom of Corinth. As such, Jason is eager for this arrangement to obtain.
Jason accepts the offer. He tells himself that this is in the best interest of both himself, Medea, and their children. He tries to convince Medea as well, telling her that as prince, and king of Corinth he would be in a much better position to provide for and protect both Medea and their sons.
Medea views this development as a crime against her and her children, as she is herself a person of royal and divine lineage. Her father was both a king, and the son of a god, Helios.
Medea sees Jason’s decision as both criminal, and a deep personal betrayal, because she risked her own life, she lost her position in her family, giving up her rights as a princess to aid Jason in his time of trial, escaping with him across the seas so that she might be his wife.
She risked everything for him out of love, and bore his children; now he proposed to set her aside, and make her the “second-wife,” essentially a servant to the princess Glauce.