Medea is a foreigner and considered a witch, and she was only accepted into the society due to her marriage to Jason. Because, I feel at heart, Jason knows what he's doing will hurt Medea (I think he needs to have at least one redeeming quality), he's asked for her not to be exiled or banished. He probably also wants to still be able to see his children. It would be much easier for Jason to exile her, and not to have anything to do with her or have to deal with any of her backlash, but because they've shared a past--not to mention a bed--and she's done many things to help him, he feels that it's only fair.
Medea, as you can probably tell, is beyond angry about what Jason is doing and she fights with him. Jason was probably hoping she wouldn't fight, but knowing her fiery spirit, he knew she would fight. He's using his power he has over her and the fact that she is a foreigner to scare her.
I hope this helps. If you have any other questions, post them here and I'd be happy to help.