The US does offer high quality sex education to children and young adults, but the real challenge is fully implementing the programs. Some regions or schools are vehemently opposed to sex education, and are often the regions with the highest instances of teenage pregnancy and STDs. Quality programs are worthless if they aren't put into full use.
While not necessarily the best in sex education, the United States' sex education approach in most public school districts is superior to that of most developing nations, which makes up the majority of the world population. Therefore, relatively speaking, the U.S. has quality sex education. Most students learn about basic contraceptive use and are warned about HIV/AIDS, which has been on the decline over the past 20 years.