Most people accept the idea that unmarried young adults are sexually active. Birth control is readily available as is the morning-after pill. HIV cases are down. Television and movies portray casual sex as normal and expected. Society is more tolerant of sexual activity in all its forms than ever before. But, researchers at the Centers for Disease Control and Prevention who keep track of these things report that for the past 20 years, the percentage of high schoolers having sexual intercourse has dropped from 54% to 40%. Research, done at San Diego State University, reported that people in their 20s are more likely to be abstinent than Gen X-ers (born before 1980 but after baby boomers) were at the same age.