Men have evolved to have a strong in-group preference, which I think is because Nature makes the presupposition that my characteristics are the "Best"(It has to with regards to males otherwise why bother living and competing?). Women did not evolve to have an in-group preference because the survival of their lineage comes down to selecting for the best male, which is as of yet still undetermined until male competition resolves itself. You could also presume that they are the offspring of the best male, so women could seek men with the same characteristics as their fathers, who is roughly similar to them, and that might create an in-group preference within the women the same as it does in men. In-group preference is a lot stronger in men than it is in women, but it does depend on the environment. Women will betray the in-group if a more dominant male phenotype comes along, but will remain loyal in stable environments. So proof of superiority is necessary, Civilization is Stability that arises from one male phenotype being dominant. Men will betray the in-group within the context of conquest, but no presumption that these outside females are superior is being made, just that they need to be conquered to prevent the rival lineage from continuing.