"In the modern perspective, we don't murder spouses who are unfaithful, we divorce them."
Depends on the culture. Pleanty still kill unfaithful wives. Or, about as often disfigure them with acid or other means. Ethics aren't universal. And since the Bible itself has death as punishment for adultery, that's a big chunk of the planet nevermind other religions like Islam and Hinduism who may feel similarly.
20 “‘Do not have sexual relations with your neighbor’s wife and defile yourself with her.
29 “‘Everyone who does any of these detestable things—such persons must be cut off from their people.
- Leviticus 18
'Cut off' has been interpreted differently over the centuries. Some claim it means exile, others death. I tend to be of the 'exile' camp since punishments for death are spelled out explicitly without any need for euphamisms. Unfortunately, it's stated again in chapter 20 much more clearly:
10 “‘If a man commits adultery with another man’s wife—with the wife of his neighbor—both the adulterer and the adulteress are to be put to death.
- Leviticus 20
In Islam, it's different:
The [unmarried] woman or [unmarried] man found guilty of sexual intercourse - lash each one of them with a hundred lashes, and do not be taken by pity for them in the religion of Allah , if you should believe in Allah and the Last Day. And let a group of the believers witness their punishment.
- Sura 24:2
Difference is in Judaism, adultery is only when a married woman has an extramarital liason. In Islam, it's either the husband or the wife. (i.e. a Jewish husband having a liason with a single woman isn't commiting a sin-crime. But a wife having a liason with a single man is. Islam regards both as equally guilty, which is an opinion I'd actually agree with.)
I've concluded that religious punishments for adultery is because of how overly-valued hereditary is in such systems. Since inheritance comes into play based on who you're related to, and since Judaism at least treats the children of an adulterous trist as legitimate heirs, forbidding such things makes sense form an economic perspective.
Outside religion, "Promiscuity is a citizen's duty." as Huxley said in "Brave New World." And interestingly enough, that sentiment has been confirmed by neuroscientists and researchers as far back as the 70s:
"Extramarital Sex
I also examined the influence of extramarital sex taboos upon crime and violence. The data clearly indicates that punitive-repressive attitudes toward extramarital sex are also linked with physical violence, personal crime, and the practice of slavery. Societies which value monogamy emphasize military glory and worship aggressive gods.
These cross-cultural data support the view of psychologists and sociologists who feel that sexual and psychological needs not being fulfilled within a marriage should be met outside of it, without destroying the primacy of the marriage relationship."
Article: Body Pleasure and the Origins of Violence