I agree that Kate is an idiot. Her boyfriend is not going to marry her, he just wants to keep the relationship going the way it is, with him getting laid with no strings attached.
If you get the milk for free, why buy the cow?
Kate is trying to guilt shame him into doing something not in his best interest.
Do you feel good guilt shaming someone to do something you want? Do you really think that is a good basis for a male-female relationship to be built on?
If Kate thouht she could do better she would dump his can and nobody would look at her and say how childish or how selfish of her to leave him.