Infidelity

People who form a couple, or who are married, very often have an agreement that they will only have sex with each other, and with no one else. If one of the two has sex with someone else, this is called infidelity.
Infidelity is common: About half of the men, and a quarter of women reported having an extra-marital affair, according to the Kinsey Reports, done in the 1950s.
In many countries, where religion is still present in everyday life (mostly in Africa, the Middle East, and Asia), being untrue to the one you love is a criminal offense. In most European countries, and the Americas, it is no longer a crime punishable by law.