human sexual behavior from an evolutionary perspective, homework help

The textbook discusses several aspects of human sexual behavior from an evolutionary perspective. Please study those sections before attempting to respond to this RT.

Some surveys are showing that 98% of sex acts in humans is Recreational in nature (for fun), and only 2% of sex acts in humans is specifically for Reproduction (to have offsprings). Why would recreational sex have evolved in humans and how recreational sex gives humans an evolutionary advantage? Please explain in detail.

(Please note that recreational sex is not a new phenomenon in human history. Historical records show that long before the advent of modern birth control methods, people were engaging in recreational sex. In the context of the question, recreational sex also IS NOT the same as premarital or extramarital sex. Majority of faithful, married and committed couples engage in recreational sex. Your response must focus on biological reasons.)