infidelity. used only in regard to cheating/affair

The Ngram viewer reports that the word "infidelity" has undergone a reduction in use by a factor of ten since the Golden Age of Infidelity peaked around 1830. The word literally means unfaithful, and back in the day, it was common to use the word to mean the state of being an infidel, i.e., living without the acceptance of God. (The "right" God, of course). Consider Herman Hooker's 1836 tract Popular Infidelity* in which he describes people who

... are without God in the world; that is they are without influence from [H]im .... They are infidels.... There can be no living after [their] manner ... and this again, is infidelity.

This usage has fallen away, and although you can still find a few uses in the sense of a corrupted version of some ur-text, modern usage is heavily devoted to the sexual transgressions of married people.

In most jurisdictions, not listening to your spouse is a misdemeanor, while killing her with intent is a felony. Neither of these is (modern) infidelity, which requires betrayal involving a third party.

*By which he didn't mean that everybody liked infidelity, just that it was widespread among the populace.