Many religious have strict limitations on what is considered sexually acceptable and unacceptable behaviour. Therefore, teaching matters of sex in a secular way is an affront to many religious principles. As most Western school systems are secular, sex education should not be taught in schools.
If the government announced that Christian prayer would be mandatory in all public schools, there would an outcry. Parents would be climbing over each other to denounce the move. Those that hold contrasting religious beliefs would denounce the move for teaching children ideas that directly violate their religious freedom. Those that were agnostic or atheist would have some form of the same argument. But contraceptives and same-sex marriage are both topics that several religions disapprove of. Teaching children about these matters in a way that doesn’t clearly define the moral, ethical and religious perils of these lifestyles is a direct violation of many people’s spiritual beliefs. The government does not have the right to violate parents’ spiritual beliefs in this way, just as it does not have the right to impose Christian prayer on students. Sexual education has always been conducted at the parents' discretion. Keeping it out of schools ensures that parents of different faiths can educate their children in a way that is sensitive and compatible with their beliefs.
“A man will leave his father and mother and be united to his wife and they will become one flesh”- Genesis 2.24 Sex is a part of a relationship. This is evident in the bible and presented as an integral part of marriage. Teaching students about sex and relationships in school is not an assault on spirituality but part of preparing them to enjoy sex in a safe and healthy way. Deeply religious people can, and should, enjoy sex. Therefore, teens should be educated on matters of sex and relationships as part of their formal education to prepare them to live healthy, well-rounded, spiritual lives.  Also, the parallels with enforced prayer do not translate. Sex education is also a public health matter. Failure to adequately educate children on matters of sexual health has a direct impact on their wellbeing and the wellbeing of others. It is therefore irresponsible, from a public health perspective, to not teach sex education in schools and ensure all teens and children receive access to the information necessary to protect themselves and their peers.
[P1] Religions have strict rules governing sexual behaviour. [P2] Therefore, it is integral to many religions that their children are taught sex education in an acceptable way. [P3] Schools teach sex education in a secular way. [P4] This is unacceptable to many religions. [P5] Therefore, sex education should not be taught in schools.
[Rejecting P5] But sex education is not solely a religious matter, it is also a public health matter and failure to educate children leaves them vulnerable to disease and infection.