>>281114623
No, historical harems worked because laws protected men, while women were used as currency. So if a wife cheated on her husband, she was immediately punished, serving as an example to other women not to try something like that, making it easier for them to accept life in a harem.
In a post-apocalyptic world, there are no laws, so everyone has to watch their own back. Even a friend might betray you if they have to decide between you and their own life. For example, at any moment your harem of just two women could betray you while you sleep and steal your resources if they think they can be better off without you or if there is someone with better resources. Think about it, there is nothing that can stop them.