Every child is different
Children and teenagers develop at different rates. You cannot apply a one-size-fits-all approach to sex education.
Why are we teaching biological instinct? If you are heterosexual, you have a biological instinct to find a potential mate and reproduce. If you are homosexual or lesbian, your urges also instruct you to find a sexual partner. We don’t need to teach these instincts. They are simply there.
No other species requires formal sexual instruction between adults and their offspring. Procreation is a natural, biological instinct. Humanity managed perfectly well before the introduction of sex education. Our ancestors lived healthy, fulfilled lives without formal sexual instruction.
Sex education does not just mean teaching the biology of sex, which is instinctual. It deals with issues of consent, social norms, disease prevention and healthy living. It is true that humans have survived for millennia without formal sexual instruction. But sexually transmitted diseases were rife, sexual assault has been common and as a race, we have been historically intolerant of the LGBTQ+ community. Sex education can help with all of these things. Just because humans have survived for millennia, doesn't mean that we can't aspire to be better. Sex education is part of that aspiration to create a better society for everyone.
[P1] Reproduction is instinctual. [P2] Instincts are innate and do not require teaching. [P3] Therefore, sex education does not need teaching.
[Rejecting P1] Sex education isn't just about reproduction.