Young men today comprise half of the largest generation in history to enter adulthood—a generation of one billion boys and girls ages 10 to 19, or about one-sixth of the world's population. When they marry and begin to raise families, these young people will have an enormous impact on worldwide health, fertility, and population growth (4).

Most sexual activity of young people takes place within marriage (159). In the developing world the majority of young unmarried people, especially young women, are not sexually active. Nevertheless, millions of young men are sexually active before marriage. Among young adults who are sexually active, sex is usually episodic, averaging a few times a month (95, 168).

Almost everywhere, the average age at first marriage has been rising, while the average age of sexual initiation is getting progressively younger (159, 168). As this gap widens, young people have more sexual partners before marriage, putting themselves at greater risk for pregnancy and STDs, including HIV/AIDS (51, 148, 159). Earlier sexual initiation may be explained partly by the decline in the age at which puberty begins in boys and girls. For boys, puberty now begins between the ages of 9 and 14 (224).

The influence of testosterone, a hormone that motivates people to engage in sex, is an important and often overlooked factor in the sexual behavior of young men. During men's adolescence and into their early 20s, testosterone levels are very high and account for much of their strong sexual urge (145, 247).

Social pressures as well as physiologic changes encourage young men and boys to take sexual risks, often to the detriment of their own health and especially their partners' (95, 159). Young men are more likely than young women to be sexually active, to have multiple partners, and to have intercourse with casual acquaintances. Their sexual behavior reflects a double standard that exists in most societies—accepting premarital sexual activity by young men while punishing such behavior by young women (159).

Unmarried men generally become sexually active at a younger age than unmarried women do, and these young men have sex more often (24, 159, 168). The average age of first sexual activity varies by country, but most young men have had sex well before age 20. According to Young Adult Reproductive Health Surveys (YARHS) conducted in Latin America, the mean age of young men's first intercourse ranged from 13.9 years in Jamaica to 16 years in Santiago, Chile (170). In these surveys sexually active young men report having sex two to five times per month, although these self-reports may be exaggerated (168).

A 1995 study of unmarried, urban youth in Guinea found the mean age of first intercourse for males was 15.6 years (95). About half of the sexually active young men reported having sex one to three times in the previous month, and the other half, more frequently (95). In Thailand more than half of the boys surveyed reported having sex by age 18, often first with a prostitute (293).

Limited knowledge, limited protection. Most young men have a lot to learn before they can become responsible sex partners. Millions of young, unmarried men are having sexual relations but know little about the consequences. Few young men, for example, understand fertility or the menstrual cycle (95, 168). Many think, mistakenly, that pregnancy cannot occur if their partner is a virgin (95) or that a woman is most fertile during menstruation (168). Furthermore, many young men do not know about modern contraceptives or where to get information and services (159). Even if they do know of contraceptive methods, many believe common misconceptions—for example, that contraception causes infertility (95).

In the YARHS the percentage of young men reporting use of contraception at first premarital intercourse ranged from 11% in Jamaica to 33% in Costa Rica. Condoms were the most common choice of men in the 14 surveys as a whole. In Guatemala City, Rio de Janeiro, and São Paulo, the Pill was the most commonly used method. In Santiago and Mexico City the rhythm method was used most often (168). In Romania 35% of all sexually experienced young men used some contraception at first premarital intercourse. Condoms and withdrawal were the most common methods (226). In the Romania, Jamaica, and Mexico City YARHS, young men's most common reasons for not using contraception at first intercourse were that sex was unexpected and that they did not know about any method (169, 170, 226).

In Guinea, among sexually active young men ages 15 to 19 who were in school, 31% said they had used contraception at first intercourse. Among similar men not in school, 22% had used a method. Condoms were the most common method for both groups (95).

Contraceptive failure is common among young adults because they do not understand how contraceptives work or do not have the skills and practice to use them effectively (4). Even if young adults do seek to learn about contraception and to use it correctly, they are often discouraged by unfriendly, even rude, treatment from providers who disapprove of sexual activity among unmarried youth (4).

Exposure to HIV/AIDS and other STDs. About half of all people infected with HIV are younger than age 25 (4). Given the slow progression from initial HIV infection to AIDS, the high incidence of AIDS among men in their 20s indicates that many contracted HIV before age 20 (159). The younger that people are when they become sexually active, the more likely they are to have multiple sexual partners. Thus they face greater risk of exposure to STDs, including HIV (4, 159). Most sexually active young men know little about STDs or how to prevent them (159).

Even when young men do know about STDs, inexperience or denial as well as cultural pressures can make them take unnecessary risks. For example, in Brazil, Ecuador, and Chile, almost all the young men surveyed reported that they had heard of HIV/AIDS. About 80% knew that a person can be infected with HIV but show no symptoms. Despite this knowledge, most did not think that they faced much risk for HIV infection, even though they were sexually active (168).