Which term best describes marital infidelity that often leads to legal issues?

The term that best describes marital infidelity leading to legal issues is adultery. Adultery refers specifically to a married individual engaging in sexual relations with someone other than their spouse. This act can have significant legal ramifications, particularly in divorce cases, where it may impact the division of property, alimony, and custody arrangements. Courts may consider adultery when determining the nature of the marriage's breakdown and the conduct of the parties involved.

In contrast, bigamy involves being married to two individuals at the same time, which is illegal in most jurisdictions but doesn't necessarily stem from infidelity. Cohabitation refers to individuals living together without being married, which is not directly tied to marital infidelity. Separation refers to a situation where married couples choose to live apart, which may or may not involve issues of infidelity. Therefore, among these options, adultery most accurately encapsulates the concept of marital infidelity with legal implications.