Final answer:
Cohabitation is when a couple lives together in a sexual relationship without being legally married, and it has become more common as societal attitudes shift.
Step-by-step explanation:
Living together in a sexual relationship when not legally married is known as cohabitation. This arrangement is chosen for various reasons, including spending more time together, saving on living costs, or as a trial run for marriage. Statistics show a significant increase in cohabitation in recent years, with many people viewing it as a step toward marriage, and some not planning to marry at all. Cohabitation has implications for the institution of marriage and family structure and is contributing to the changing patterns of forming relationships and families.