Females are only sort of loyal as long as the male can protect them and their offspring and they will find opportunities to get dicked by other males any chance they get in the meantime for the gene pool.
Look at the world of our closest cousins if you want scientific proof. Humans are obviously more complicated but adhere to the same principles.
Tribe is invaded, male population killed, females will either die or breed with the new protectors until the next invasion. Women are wired to be less bonded to us than we are to them which is why they can discard a man easily for perceived better genetic options and resources. They have had millions of years of practice.