Why are people so against abstinence education

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Background and context

The debate between "comprehensive sex education" (also just called "sex education") and "abstinence-only education" is long-standing in the United States, and exists in many other societies around the world that are split between more sexually progressive groups and generally those that oppose pre-marital sex.
Comprehensive sex programs teach teenagers, mostly in high school, a range of information related to their own sexual anatomy, the act of sex, the use of contraceptives, and the risks of pregnancy and STDs associated with having sex.
Abstinence-only programs, conversely, are much more conservative and limited in scope, teaching abstinence from sex, usually until marriage, as a means of avoiding the risk of pregnancy and STDs and of enjoying other considered benefits such as a more unique sexual bond with one's ultimate partner.

Many questions frame this debate: Is abstinence a good message? Is teen sex and pre-marital sex wrong? Or, is teen sexuality and possibly sex natural and acceptable? If the goal is to reduce sex rates among teens, does abstinence-only succeed? And, what about comprehensive sex education? If the goal is to reduce the spread of STDs among teens, is one approach better than the other? And, what about for the goal of reducing teen pregnancy? Are condoms effective at lowering these risks, and are comprehensive sex education programs doing a good job of promoting them? Is comprehensive sex education the job of schools, or should this role be reserved for parents? What about for kids that do not have parents, or whose parents are not responsible care-givers?

As is evidenced by these questions, one of the unique elements of this debate is how it brings together so many fields of study, including sexual culture, marital culture, faith, human instincts, pregnancy, disease, parental-school-state roles, and many other issues.

See Wikipedia's articles on comprehensive sex education and abstinence-only for greater background and history