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What it means us the guy finds something more to his liking about the other woman. It is never about 'better' and always a case of more compatible or fit with him and his lifestyle and / or current needs.
A further point is when it moves beyond casual dating, both partners should agree on exclusivity. A guy should never have 2 girlfriends or a side chick. In my mind having 2 girlfriends is cheating and having a side chick means you're hiding one and dating another, which is even worse. In the case a guy has 2 girlfriends and he leaves one to be with the other - don't take it personally - he's not a good guy and you're better off without him in your life.