Like united as in sharing the same governmental structure (or lack thereof sometimes), freedom to move and travel anywhere, and probably more or less similar ideals for such a thing for actually work.
There still obviously would be things to fight over and probably some amount of small-scale civil conflict. There would also still probably be areas with with similar cultures, but with softer and more grey edges and mixing.
This is also more or less just the Marxist ideal of things, I have slightly different ideals personally. Mostly that there does need to be a fairly defined state and governmental system to maintain socialism/communism, help organize large-scale resource allocation and transport on a global scale, and provide structure for civilization-scale projects like progressing human knowledge and science, space travel and exploration, etc.
I think you might over-estimate how common that would be if such hate and opinions were not supported by the state or at least not ignored by the state, but it is an understandable concern, but I see a few possible arguments against it.
the lack of such freedom of mobility and movement of culture would let cultures mix and have more interaction, which has been shown to increase acceptance of different cultures, and reduce hate.
there will almost always be cultural differences, and dislike between groups, but especially without class struggles it will be less common for them to elevate to the levels of lynchings, and outright conflict. Hell, even just looking at the US, it has a decent amount of separate cultural regions but not much conflict based on that. It is mostly interpersonal conflict, class-based, or from reactionaries to minorities.