For many religious people, raising their children in their faith is an important part of their religious practice. They might see getting their kids into heaven as one of the most important things they can do as parent. And certainly, adults should have the right to practice their religion freely, but children are impressionable and unlikely to realize that they are being indoctrinated into one religion out of the thousands that humans practice.
And many faith traditions have beliefs that are at odds with science or support bigoted worldviews. For example, a queer person being raised in the Catholic Church would be taught that they are inherently disordered and would likely be discouraged from being involved in LGBTQ support groups.
Where do you think the line is between practicing your own religion faithfully and unethically forcing your beliefs on someone else?
Yes, but people learn about it late (if at all), and we end up with lots of adolescents getting STIs/pregnant/etc.
America has a problem with sex ed because people don’t learn about safe sex; many still learn abstinence only. This doesn’t stop STIs nor teen pregnancies, it doesn’t stop SA, it doesn’t stop myths about men and womens reproductive systems from proliferating, it just defers the problem of educating people until later. Basically, America’s sex ed is to avoid teaching people about sex, then hope they suddenly know how to have safe sex when they’re 18 because they’re 18.
Likewise, deferring learning about cults until they’re 18 doesn’t stop people from getting indoctrinated, it just expects 18 year olds unfamiliar with cult tactics to suddenly be immune to cult tactics because they’re 18.