Is it loss?
This has evolutionary reasons: For men it is bad if your wife is wrong with other men, as they get so children who are not by the man himself (including his DNA) Nevertheless, feed through. For women, it is particularly bad if your husband has emotional interest to other women, as they risk that the man can be alone in the long run with the children. It is also assumed that the revenue of civilizations also flashed gender diseases, or became a problem, which ultimately has enforced the monogamy in width parts of the earth. In addition, in most cultures, the idea has been enforced that one cheats mutually nixed, because that's just a massiveHe is trust.
As a rule, this is associated with allegations and lies. From the risk of suspecting the partners diseases, not to mention. This is a massive trust breakage.
Who is like a foreign skin, can communicate that the partner exactly as well. Then he has at least the opportunity to decide himself if he still wants or not this relationship under the given signs or not.
I think in most cases it is revealing. But the woman does not participate passively usually. Men like to be ashamed especially if there are other from the puncture.
I think it's only bad when children are involved between this tension.
Inner betrayal / relationship is just just modern today
Something like that you do not take the partner, that's a trust break. Loyalty is usually partnership and mutual trust.
In my opinion, it is because it does not cheat a principle of a relationship that has been promoted to us